Detection of CPD from signals captured at remote PHY device

ABSTRACT

A method and apparatus is disclosed for enabling synchronous capture of forward and return signals at a remote PHY device for detecting CPD. The remote PHY device provides the forward signal to and receives the return signal from a coaxial cable plant. The return signal contains an actual CPD signal generated by an interaction between the forward signal and a CPD source in the cable plant. The forward signal propagates from the remote PHY device to the CPD source and the actual CPD signal propagates from the CPD source to the remote PHY device all within a round-trip interval. The method comprises: capturing the forward signal from an output associated with the remote PHY device; generating from the forward signal a simulated CPD signal having frequencies in a return channel; band-limiting the simulated CPD signal to the return channel to produce a reference CPD signal; supplying the reference CPD signal to the remote PHY device; in the remote PHY device, receiving the reference CPD signal in the return channel; and (f) in the remote PHY device, capturing the return signal from the return channel over a capture duration of at least the round-trip interval. The reference CPD signal is used to detect the actual CPD signal from the return signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.16/125,837, filed Sep. 10, 2018, which claims the benefit of: U.S.Provisional Application No. 62/681,188, filed Jun. 6, 2018, which isincorporated herein by reference; and U.S. Provisional Application No.62/683,417, filed Jun. 11, 2018, which is incorporated herein byreference. This application is related in subject matter to U.S. Pat.No. 10,158,423, issued Dec. 18, 2018, naming the same inventor, and isincorporated herein by reference.

BACKGROUND OF THE INVENTION Field of Invention

The present invention relates generally to the detection of impairmentsin coaxial cable plants of hybrid-fiber coax (HFC) communicationnetworks employing remote physical layer (PHY) devices, for example, ina Converged Cable Access Platform (CCAP) architecture, and moreparticularly to apparatus and methods that aid in the detection ofcommon path distortion (CPD) in the coaxial cable plants of suchnetworks.

Background Art

One type of impairment in an HFC network is common path distortion(CPD). This impairment is nonlinear and involves the intermodulation(IM) of downstream or forward path (or “forward”) signals occurring atvarious network components or elements in the “common path” of thenetwork, such as amplifiers, subscriber taps, connectors, contacts,terminations, etc. The IM of the forward signals is the result of aso-called “diode effect” caused by, e.g., corrosion of theabove-mentioned components and elements in the network. Such componentsand elements may also exhibit nonlinear behavior from mechanisms otherthan corrosion, such as, e.g., ferrite materials, oil films and othercontaminants, thin insulators, mutual contact of different metals, oractual diodes elements. A very good explanation of the causes of CPD isfound in an article by Bharat (Barry) Patel, entitled, “Common PathDistortions Explained,” dated Feb. 3, 1998, pp. 1-29. CPD areintermodulation products of the forward signals in the upstream orreturn path spectrum of the HFC network. CPD amounts to noise that canadversely impact data transmission in the return path. Accordingly, thetask of finding and eliminating sources of CPD is a goal to ensurequality of data and other services in the upstream or return path (or“return”). A component or element (or an affected part thereof)exhibiting nonlinear behavior (or a nonlinear response) is sometimesreferred to herein as a “CPD source” or “source of CPD.”

In recently specified and implemented HFC networks, the CPD impairmentproblem is expected to become more difficult to solve due to severalfactors. First, under the Data-Over-Cable Service InterfaceSpecifications DOCSIS® 3.1, Physical Layer Specification,CM-SP-PHYv3.1-I13-171220, published by Cable Television Laboratories,Inc. (CableLabs®), Dec. 20, 2017, which is incorporated herein byreference (hereinafter “DOCSIS 3.1 Specification”), the bandwidth of theforward spectrum in HFC networks could increase up to 1794 MHz. Withincreases in bandwidth, the total energy of the forward signal will alsoincrease, and, as result, the level of CPD in the return spectrum willalso increase. Second, there is a migration away from analog channels toall digital channels in the forward spectrum. In the case of analogchannels, CPD generally appears as discrete harmonics spaced 6 MHz apartand is thus relatively easy to detect, while, in the case of QAM orother digitally modulated signals, CPD appears as flat noise spread overthe full return spectrum, making it more difficult to detect. Third,under the most recent DOCSIS 3.1 Specification, the bandwidth of thereturn spectrum in HFC networks will increase up to 685 MHz byimplementation of Full Duplex (FDX) technology. With these increases inbandwidth, the number of different order nonlinear IM products generatedat a CPD source will increase, which may impact data signals at least inthe return spectrum of FDX. See further discussion of these issues inprovisional application No. 62/618,246, filed Jan. 17, 2018.

Methods of detecting CPD have been developed over the years. One suchmethod is to use a spectrum analyzer to monitor the return spectrum forIM products caused by CPD. Such a method is adequate for legacy HFCnetworks carrying a large number of analog channels. In these networks,CPD looks like a number of discrete 6 MHz harmonics (for NTSC frequencyplan). But, in the case of an all-digital network (e.g., QAM and/or OFDMsignals), CPD in the return spectrum looks like flat noise and is noteasily distinguishable from additive ingress noise. Another limitationis that a spectrum analyzer does not allow one to identify multiplesources of CPD, which is not an uncommon impairment scenario. Also, aCPD source cannot be directly located using a spectrum analyzer.Further, a spectrum analyzer is unable to detect very low level CPDdistortion products. The ability to detect very low level CPD isdesirable because it allows one to identify CPD sources early in theirdevelopment, before they impact signal quality, thus making it possibleto implement a proactive network maintenance (PNM) program. Also, verylow level CPD detection is useful to identify CPD sources that mayimpact signal quality intermittently (which is a common occurrence). Inlatter case, low level CPD may increase dramatically for a moment due tomechanical (e.g., wind), temperature, moisture and other environmentalfactors.

Another known method of detecting CPD is known as the Hunter® Xcor®system available from Arcom Digital, LLC, Syracuse, N.Y.(www.arcomlabs.com). This system is described in the following patentdocuments: U.S. Pat. No. 7,584,496 to Zinevich and U.S. Pat. No.7,415,367 to Williams, both of which are incorporated herein byreference. The idea behind this system is to use cross-correlationdetection, where a reference CPD signal is generated at a headend (or inthe field) from a forward signal and then cross-correlated with anactual CPD (echo) signal received from the coaxial cable plant of an HFCnetwork. Advantages of this system are its ability to detect low levelCPD signals and multiple CPD signals at different locations, due to goodsensitivity and time delay resolution. This technology is now widelyused in HFC networks across the United States and in many othercountries. It has proven to be very effective in the early detection andlocation of CPD sources, which is important for a PNM program.

In a modern HFC network with CCAP and remote PHY architectures, theforward and return signals will be generated and received directly inthe field by remote PHY devices. Thus, both forward and return signalswill not be physically accessible at the headend of the network, whichcan limit the usefulness of the Hunter® Xcor® system. The followingindustry specifications have been prepared for CCAP and Remote PHYarchitectures: Data-Over-Cable Service Interface Specifications DOCSIS®3.1, CCAP™ Operations Support System Interface Specification,CM-SP-CCAP-OSSIv3.1-I07-160602 (Jun. 2, 2016); and Data-Over-CableService Interface Specifications, DCA-MHAv2 Remote PHY Specification,CM-SP-R-PHY-I05-160923 (Sep. 23, 2016). Both specifications areincorporated herein by reference.

A method and system of CPD detection in a CCAP architecture has beenproposed using synchronous capture of a forward OFDM symbol andtime-domain samples of an actual return CPD signal. Here, a referenceCPD signal is generated from samples of the captured OFDM symbol. Thesesamples are then cross-correlated with samples of the actual return CPDsignal. This method is disclosed in U.S. Pat. No. 9,826,263 to Zinevich.To achieve good detection sensitivity (or processing gain) with thismethod, a relatively large amount of data over a relatively long periodof time must be captured. For example, the accumulation time at thecross-correlation detector is about 100 milliseconds or more fordetection of CPD under the noise floor. This means that at least 5000OFDM symbols should be captured. The symbols are captured at a CCAP core(see CCAP specification), which has a capture rate of about 10 or lessOFDM symbols per second. Thus, the total time required for capturing5000 symbols is about 8-10 minutes and the total size of the captureddata (OFDM symbols) will be about 160 MB. And, the size of the CPDsignal data will be many times more due to the large sampling rate ofreturn signals (around 400 MHz). As a result, the total size of captureddata for CPD detection at one node will be around 1 GB, which is still alarge amount of data to handle. Another concern with this method/systemis that the capturing of I and Q samples of the return signal must betriggered or synchronized with the capture of the forward symbols. Sucha trigger or synchronization mechanism may not be available in CCAPnetworks in accordance with DOCSIS specifications (i.e., it is notrequired). Thus, there is no guarantee that sync capture of I and Qsamples will be possible in a CCAP/Remote PHY platform.

The use of test equipment modules contained within host network devices,such as bi-directional amplifiers, legacy fiber nodes, Remote PHY nodes,or other remote PHY devices, have been proposed. Such modules arereferred to in the Remote PHY Specification CM-SP-R-PHY-I05-160923(cited above), in Appendix I, at Section 1.2, Hardware Module in theNode (p. 180). There, it is stated that: “Test equipment vendors maydevelop modules that will be deployed within a node that supports theR-PHY architecture that performs the same function as the equipment thatwas previously deployed in the headend. Since the module is located inthe R-PHY Node, the same telemetry and control channels can be used.” Acommercially available module of this type (intended for amplifierenclosures) is a Hitron CMS-02 Embedded Spectrum Analysis Module,supplied by Hitron Technologies Americas Inc., Centennial, Colo.(www.hitron-americas.com/product/cms-02). This and similar modulesutilize a DOCSIS-compliant cable modem (CM) for communicating (or“backhauling”) measurement data to a PNM server. CM operation requires asizable amount of power from the host device (e.g., remote PHY node).Also, such DOCSIS modems are costly. Finally, a test equipment moduleusing a CM has a relatively large size and thus occupies a relativelylarge space within a host device. Thus, the deployment of such testequipment modules poses significant challenges.

Accordingly, a need exists in a modern HFC network for a convenient andreliable way to achieve synchronous capture of forward signals andactual CPD echo signals from remote PHY devices in a CCAP architecture(for use in detecting CPD and locating CPD sources). The approach shouldbe compatible with most if not all remote PHY devices, have minimalimpact on such devices, and should not require large amounts of captureddata for detection and ranging.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the present invention to overcome theproblems and limitations associated with the prior art and to fulfillthe aforementioned need.

It is another object of the present invention to provide apparatus andmethods that achieve synchronous capture of forward and return signalsat a remote PHY device, for the purpose of detecting CPD and locatingits source(s).

It is a further object of the present invention to provide apparatus andmethods that achieve synchronous capture of the forward and returnsignals at a remote PHY device, without modifying or otherwise impactingthe remote PHY device.

It is still another object of the present invention to provide apparatusand methods for synchronous capture of forward and return signals (forCPD detection) that are compatible with most if not all remote PHYdevices.

It is still a further object of the present invention to provideapparatus and methods for synchronous capture of forward and returnsignals (for CPD detection) that do not require any or significant powerconsumption.

It is yet another object of the present invention to provide apparatusand methods for synchronous capture of forward and return signals (forCPD detection) that do not generate large amounts of captured data forCPD detection and ranging.

It is yet a further object of the present invention to provide apparatusand methods for synchronous capture of forward and return signals (forCPD detection) that utilize an existing return channel of the remote PHYdevice for communicating forward and return captured data to a CPDdetection core or server.

It is still yet a further object of the present invention to provideapparatus and methods for synchronous capture of forward and returnsignals (for CPD detection) that communicate captured data to a CPDdetection core or server without the use of a cable modem.

These and other objects are attained in accordance with the presentinvention, wherein there is provided, in one embodiment, an apparatusfor enabling synchronous capture of a forward signal and a relatedactual CPD signal at a remote PHY device, for the purpose of detectingCPD and locating its source. The remote PHY device is coupled to anddelivers the forward signal to a coaxial cable plant. The actual CPDsignal is generated by an interaction between the forward signal and aCPD source in the coaxial cable plant. The forward signal propagatesfrom the remote PHY device to the CPD source and the actual CPD signalpropagates from the CPD source to the remote PHY device all within around-trip interval. The actual CPD signal is captured in the remote PHYdevice in a return channel during the round-trip interval. The apparatuscomprises a coupling element, a CPD simulator circuit, and a filter. Thecoupling element is coupleable to the remote PHY device for capturingthe forward signal from the remote PHY device during the round-tripinterval. The CPD simulator circuit is coupled to the coupling elementand generates from the forward signal a simulated CPD signal having amultiplicity of frequency components. The multiplicity of frequencycomponents includes frequency components within the return channel. Thefilter substantially selects from the multiplicity of frequencycomponents the frequency components within the return channel, toproduce a reference CPD signal. The filter is further coupled to thecoupling element to supply the reference CPD signal to the remote PHYdevice. The reference CPD signal is received by the remote PHY devicewithin the return channel. Accordingly, the reference and actual CPDsignals are available from the return channel for use in detecting theactual CPD signal and locating its source in the coaxial cable plant.

In more specific embodiments, the filter may includes or be a bandpassfilter having a passband that substantially matches the return channel.The return channel is preferably an out-of-band return channel, such asa narrowband digital return (NDR) channel (further discussion below).The CPD simulator circuit may include or be a Schottky diode or, in apowered embodiment, a field effect transistor. The coupling element mayinclude a cable connector configured to connect to a complementaryconnector associated with the remote PHY device. The apparatus mayfurther include a highpass filter coupled between the coupling elementand the CPD simulator circuit. In such case, the highpass filter has apassband and a reject band separated by a cutoff frequency, where thepassband substantially passes the forward signal to the CPD simulatorcircuit and the reject band substantially rejects frequencies of thesimulated CPD signal lower than the cutoff frequency. The apparatus mayfurther include an attenuator circuit coupled between the highpassfilter and the CPD simulator circuit. The attenuator circuit is designedto attenuate the level of the forward signal as it travels to the CPDsimulator circuit and also attenuates a portion of the simulated CPDsignal that travels back towards the highpass filter. The reference CPDsignal, as received by the remote PHY device, preferably has a signallevel in a range of about 0 dB to about 5 dB relative to an expectedmaximum level of the actual CPD signal captured by the remote PHYdevice. The entire apparatus is preferably enclosed in a housing havinga cable connector configured to physically and electrically connect to acomplementary connector associated with the remote PHY device.

In another embodiment, the apparatus is an active device and, inaddition to the above passive elements, further comprises a pulsemodulation circuit and a time delay line (e.g., a digital time delaycircuit). In this embodiment, the pulse modulation circuit is coupled tothe bandpass filter for shaping the reference CPD signal into a pulse.The time delay line is coupled to the pulse modulation circuit. Thedelay line delays the pulse sufficiently enough such that the actual CPDsignal is captured in the remote PHY device without substantialinterference from the CPD reference signal. Preferably, the time delayline is configured to delay the pulse longer than the round-tripinterval.

In a further embodiment, the apparatus is an active device and, inaddition to the original passive elements, further comprises a pulsemodulation circuit and a phase inverter circuit (e.g., an analogtransformer). Again, the pulse modulation circuit is coupled to thebandpass filter. The pulse modulation circuit shapes the reference CPDsignal into a first pulse and shapes a second reference CPD signal(produced by the filter) into a second pulse. The phase inverter circuitis coupled to the pulse modulation circuit. The phase inverter circuitshifts the phase of the second pulse by 180 degrees. Interference causedby an autocorrelation of the first pulse, in detecting the actual CPDsignal, is substantially cancelable by interference caused by anautocorrelation of the second pulse (as will be described below).

Also within the scope of the present invention is a method of enablingsynchronous capture of a forward signal and a related actual CPD signal,at a remote PHY device, for the purpose of detecting CPD and locatingits source. The remote PHY device is coupled to and delivers the forwardsignal to a coaxial cable plant of an HFC network. The actual CPD signalis generated by an interaction between the forward signal and a CPDsource in the coaxial cable plant. The forward signal propagates fromthe remote PHY device to the CPD source and the actual CPD signalpropagates from the CPD source to the remote PHY device, all within around-trip interval. The method comprises the steps of: (a) capturingthe forward signal from an output port associated with the remote PHYdevice during the round-trip interval; (b) generating from the forwardsignal captured in step (a) a reference CPD signal, which substantiallysimulates the actual CPD signal; (c) supplying the reference CPD signalto the remote PHY device; (d) in the remote PHY device, receiving thereference CPD signal in a return channel; and (e) in the remote PHYdevice, capturing the actual CPD signal in the return channel during theround-trip interval. As a result of these method steps, the referenceand the actual CPD signals are processed as a combined return signal indetecting the actual CPD signal and locating the CPD source.

In more specific embodiments, step (a) may include capturing the forwardsignal from the remote PHY device substantially at the beginning of theround-trip interval. Step (e) may include capturing the actual CPDsignal substantially at the end of the round-trip interval. Step (c) mayinclude supplying the reference CPD signal to the remote PHY device viathe output port. Where the return channel in the remote PHY device isdefined by a noise floor, step (d) may include receiving the referenceCPD signal in the Remote PHY device at a level of about 10 dB to about20 dB above the noise floor.

The method of the present invention may further include steps involvingthe detection of the CPD and the location of its sources. It isunderstood that the remote PHY device is coupled to a headend of the HFCnetwork. The method may further comprise the steps of: (f) transmittingthe combined return signal from the remote PHY device to the headend ofthe HFC network; (g) at the headend or a device communicating with theheadend, performing a cross-correlation of the reference and the actualCPD signals of the combined return signal, to produce a correlation peakhaving a time delay corresponding to the round-trip interval; (h)detecting the actual CPD signal from the correlation peak; and (i)determining a location of the CPD source in the coaxial cable plant fromthe time delay of the correlation peak. In a more specific and preferredmethod, step (g) includes performing an autocorrelation of the combinedreturn signal to obtain the cross-correlation of the reference and theactual CPD signals. More specifically, the autocorrelation of thecombined return signal includes an autocorrelation of the reference CPDsignal and the cross-correlation of the reference and the actual CPDsignals. The method may involve the further step of substantiallysubtracting the autocorrelation of the reference CPD signal from theautocorrelation of the combined return signal. In a preferredimplementation, the combined return signal has a duration of about 100milliseconds or greater (or 800 milliseconds or greater), and step (g)above includes performing the autocorrelation of the combined returnsignal substantially over the duration of the combined return signal.

In another embodiment, and in addition to method steps (a) through (e)above, the method may further comprise the steps of: (f) performing afirst combined autocorrelation of the combined return signal, whichincludes a first reference autocorrelation of the reference CPD signaland a first cross-correlation of the reference and the actual CPDsignals; (g) repeating steps (a) and (b) to generate a second referenceCPD signal, which substantially simulates a second actual CPD signal;(h) shifting the phase of the second reference CPD signal by about 180degrees; (i) repeating steps (c) through (e) with respect to the secondreference CPD signal (phase-shifted 180 degrees) and with respect to thesecond actual CPD signal, to produce a second combined return signal;(j) repeating step (f) with respect to the second combined signal, toproduce a second combined autocorrelation of the second combined returnsignal, which includes a second reference autocorrelation of the secondreference CPD signal and a second cross-correlation of the secondreference and the second actual CPD signals; (k) subtracting the secondcombined autocorrelation from the first combined autocorrelation, suchthat the first and the second reference autocorrelations substantiallycancel each other out, and the first and the second cross-correlationssubstantially add together to produce an accumulated cross-correlationfunction having a correlation peak; and (l) detecting the presence ofactual CPD signals in the coaxial cable plant from the correlation peak.The correlation peak has associated therewith a time delay correspondingto the round-trip interval. The method further comprises the step of:(m) determining a location of the CPD source in the coaxial cable plantfrom the time delay associated with the correlation peak. In a morespecific implementation, step (b) may include shaping the reference CPDsignal into a first pulse and step (g) may include shaping the secondreference CPD signal into a second pulse.

In a further embodiment, and in addition to original method steps (a)through (e) above, the method may further comprise the steps of: (f)shaping the reference CPD signal into a pulse; and (g) delaying thepulse such that the actual CPD signal is captured in the remote PHYdevice without substantial interference from the CPD reference signal.In a more specific implementation, step (g) may include delaying thepulse longer than the round-trip interval.

In still another embodiment, there is provided a method of synchronouscapture of a forward signal and a related actual CPD signal at a remotePHY device, for the purpose of detecting CPD and locating its source.The remote PHY device is coupled between a headend and a coaxial cableplant of an HFC network and includes a forward signal transmitter, areturn signal receiver, and a diplexer having forward and return legs. Aphysical forward path is defined between the forward signal transmitterand the forward leg, and a physical return path is defined between thereturn signal receiver and the return leg. The forward signaltransmitter transmits the forward signal to the coaxial cable plant viathe physical forward path and the forward leg of the diplexer. A portionof the forward signal leaks through the return leg of the diplexer andtravels over the physical return path to the return signal receiver. Theactual CPD signal is generated by an interaction between the forwardsignal and a CPD source in the coaxial cable plant. The forward signalpropagates from the remote PHY device to the CPD source and the actualCPD signal propagates from the CPD source to the remote PHY device allwithin a round-trip interval. The method here comprises the steps of:(a) operating the return signal receiver of the remote PHY device tocapture the actual CPD signal and the leaked portion of the forwardsignal over a duration of at least the round-trip interval; (b)transmitting the actual CPD signal and the leaked portion of the forwardsignal captured in step (a) to the headend of the HFC network; (c) atthe headend or a device communicating with the headend, generating fromthe leaked portion of the forward signal a reference CPD signal, whichsubstantially simulates the actual CPD signal; (d) performing across-correlation of the reference and the actual CPD signals to producea correlation peak having a time delay corresponding to the round-tripinterval; (e) detecting the actual CPD signal from the correlation peak;and (f) determining a location of the CPD source in the coaxial cableplant, relative to the remote PHY device, from the time delay of thecorrelation peak. Note, the diplexer is a combining and splittingnetwork (e.g., such as an impedance bridge, a signal splitter, or diplexfilter).

The forward signal is generally defined by a forward signal spectrum. Ina more specific embodiment of the latter method, a further stepincludes: (g) configuring the return signal receiver of the remote PHYdevice to capture a range of frequencies of the forward signal spectrum.In a more specific embodiment, the return signal receiver has acapturing bandwidth, and step (g) includes adjusting the capturingbandwidth to maximize the range of frequencies to be captured from theforward signal spectrum. In a variation of the latter, the return signalreceiver has a capturing bandwidth centered at a center frequency, andstep (g) includes setting the center frequency of the capturingbandwidth to maximize the range of frequencies to be captured from theforward signal spectrum.

BRIEF DESCRIPTION OF THE DRAWING

Further objects of the present invention will become apparent from thefollowing description of preferred embodiment with reference to theaccompanying drawing, in which:

FIG. 1 is a block diagram of an HFC network system with CCAP and remotePHY device architecture, incorporating the present invention;

FIG. 2 is a block diagram of a passive, analog CPD generator of thepresent invention coupled to a remote PHY device via an output testport, where the CPD generator is used for generating a reference CPDsignal to be supplied to the remote PHY device in synchronism withreceipt of an actual CPD echo signal;

FIG. 3 is an image of a frequency spectrum showing the return spectrumof the HFC network with a narrowband digital return channel containingsync captured reference and actual CPD signals;

FIG. 4 is a block diagram of a more specific embodiment of the passive,analog CPD generator of FIG. 2, containing a highpass filter, anattenuator, a CPD simulator, and a bandpass filter;

FIGS. 5A-5D are a series of frequency sweeps of a forward signal beingprocessed at different points within the CPD generator of FIG. 4, whereFIG. 5A displays the signal after the highpass filter, FIG. 5B displaysthe signal after the CPD simulator (Schottky diode), FIG. 5C displaysthe signal after the bandpass filter, and FIG. 5D displays theband-limited signal (at different scale) as the output of the CPDgenerator;

FIG. 6 is a schematic diagram of the passive, analog CPD generator ofFIG. 4;

FIGS. 7A and 7B are views of two physical forms of the CPD generator ofFIG. 6, where FIG. 7A shows a housing equipped with a cable connectorand enclosing a circuit board which contains the CPD generatorcircuitry, and FIG. 7B shows the circuit board without a housing andequipped with a connector to connect to an internal test port of aremote PHY device;

FIG. 8 is an example of an autocorrelation function of a combined returnsignal containing reference and actual CPD signal signals;

FIG. 9 is a diagram of the structure of the autocorrelation function ofFIG. 8, showing contributions of different terms of the function, suchas the autocorrelation of the reference CPD signal, autocorrelation ofthe actual CPD (echo) signal, and cross-correlation of the reference andactual CPD signals;

FIGS. 10A and 10B show a pair of autocorrelation functions of thecombined return signal demonstrating reduction in noise floor byincreasing effective accumulation time of the autocorrelation, whereFIG. 10A shows an accumulation time of 50 milliseconds and FIG. 10Bshows an accumulation time of 800 milliseconds;

FIG. 11 is a flow diagram of a method of canceling the autocorrelationterm of the reference CPD signal (including a main peak at zero timedelay) from an autocorrelation function of the combined return signal,in order to unmask the cross-correlation terms, particularly at shorttime-distances;

FIG. 12 is a plot of a pre-determined complex autocorrelation functionused to cancel (or subtract) the autocorrelation term of the referenceCPD signal from the autocorrelation function of the combined returnsignal;

FIGS. 13A and 13B are before and after plots of an autocorrelationfunction of a combined return signal, produced from an actual benchtest, illustrating the effectiveness of the cancellation method outlinedin FIG. 11;

FIG. 14 is a block diagram of a second embodiment of a CPD generator ofthe present invention, showing the addition (from FIG. 2) of activecircuits including a pulse modulator and a digital time-delay line;

FIG. 15 is a series of timing diagrams for use in explaining theoperation of the CPD generator of FIG. 14;

FIG. 16 is a block diagram of a third embodiment of a CPD generator ofthe present invention, showing the addition (from FIG. 2) of activecircuits including a pulse modulator and an analog phase-shiftercircuit;

FIG. 17 is a series of diagrams illustrating operation of the CPDgenerator of FIG. 16;

FIG. 18 is a flow diagram of a method of the present invention enablingsynchronous capture of a forward signal and an actual CPD echo signal,and the correlation detection of the actual CPD echo signal and thelocation of the CPD source;

FIG. 19 is a block diagram of a remote PHY device showing remote PHYtransmitter and receiver chipsets and a generalized diplexer component,and illustrating a leakage phenomenon which forms the basis of a fourthembodiment of the present invention;

FIG. 20 is a spectrum captured by the remote PHY receiver chipset, wherethe spectrum encompasses the return spectrum of the return signals in anetwork (including actual CPD echo signal) and some of the forwardspectrum from a leaked portion of the forward signal; and

FIG. 21 is a flow diagram outlining a method of carrying out the fourthembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a block diagram is shown of a modern HFC networksystem 100, constructed in accordance with CCAP and Remote PHYarchitectures and specifications. Network system 100 comprises a headend102 which includes a CCAP core 104. System 100 further comprises adigital fiber optic cable plant 106, at least one remote PHY device ornode 108, a coaxial cable plant 110, and a number of subscriber networks112. Remote PHY device 108 is coupled at one end to fiber optic cableplant 106 and at the other end to coaxial cable plant 110. CCAP core 104contains a cable modem termination system (CMTS) core (not shown) forDOCSIS data communication and an Edge QAM core (not shown) for video.Downstream DOCSIS, MPEG video, and out-of-band (OOB) signals aretransmitted from CCAP core 104 to remote PHY node 108 via fiber cableplant 106, in the form of Ethernet or PON digital (IP) signals. RemotePHY node 108 converts these downstream signals to analog RF (radiofrequency) signals for transmission to coaxial cable plant 110 andsubscriber networks 112. Cable modems (CMs) (not shown) in thesubscriber networks 112 transmit upstream or return DOCSIS and OOBanalog signals to remote PHY node 108 via coaxial cable plant 110.Remote PHY node 108 converts the upstream DOCSIS and OOB analog signalsto digital format (Ethernet or PON) for transmission to CCAP core 104via fiber plant 106. Remote PHY node 108 is configured as an integratedoptical node and a remote PHY device (see, e.g., Section 5 of Remote PHYSpecification, cited in Background).

The term “downstream signal” or “forward signal” means signal(s)traveling in a downstream direction in network system 100 (e.g., fromCCAP core 104 to coaxial cable plant 110) and located in a downstream orforward frequency spectrum. Similarly, the term “upstream signal” or“return signal” means signal(s) traveling in an upstream direction innetwork system 100 (e.g., from coaxial cable plant 110 to CCAP core 104)and located in an upstream or return frequency spectrum. In NorthAmerican cable systems, the legacy downstream frequency band is fromjust below 54 MHz to as high as 1002 MHz or more. The DOCSIS 3.1downstream band is 258 MHz (optional 108 MHz) to 1218 MHz (optional 1794MHz). The legacy upstream frequency band is from 5 MHz to as high as 42MHz. The DOCSIS 3.1 upstream band is 5 to 204 MHz, with support for 5 to42 MHz, 5 to 65 MHz, 5 to 85 MHz and 5 to 117 MHz. See CableLabs®Data-Over-Cable Service Interface Specifications DOCSIS® 3.1, PhysicalLayer Specification, CM-SP-PHYV3.1-I10-170111 (Jan. 11, 2017),incorporated herein by reference (hereafter “DOCSIS 3.1 Specification”)

In FIG. 1, a PNM (proactive network maintenance) server 114 is a part ofor linked to headend 102. PNM server 114 is a network management systemprimarily concerned with network maintenance. Its functions includemonitoring, measuring, testing, trouble-shooting, identifying, locating,etc. with respect to: signals in network system 100; network componentsand devices operating in system 100; and impairments in system 100. PNMserver 114 stores and executes software routines and communicates withCCAP core 104 (and CMTS core) and the CMs in subscriber networks 112, toinstruct these devices to carryout maintenance functions and producemaintenance or test data. PNM server 114 communicates directly orindirectly with CCAP core 104 (or generally with network system 100) viaa virtual private network (or virtual local area network), a wide areanetwork (e.g., the Internet), and/or other suitable network link 116.Further details regarding PNM server functions and communication isprovided in the DOCSIS 3.1 Specification (Section 9) and an industryguide published by CableLabs®, called PNM Best Practices: HFC Networks(DOCSIS 3.0), CM-GL-PNMP-V03-160725 (Jul. 25, 2016), incorporated hereinby reference.

Again referring to FIG. 1, a CPD core 118 is a part of or linked toheadend 102. CPD core 118 is a monitoring system especially adapted toimplement elements or steps of the present invention. Specifically, inthe preferred embodiment, core 118 carries out the functions of CPDdetection and location via correlation processing (as described furtherbelow). CPD core 118 communicates with and may, in some implementations,be slave to PNM server 114. As shown, CPD core 118 communicates with PNMserver 114 via wide area network 116. In other architectures, CPD core118 may communicate with server 114 via a virtual private network orvirtual local area network. CPD core 118 communicates with CCAP core 104via an IP communication link 120, using, e.g., simple network managementprotocol (SNMP). CPD core 118 is preferably implemented in software,which may be installed and operated, e.g., on a dedicated CPD coreserver, on PNM server 114, or in CCAP core 104.

Again referring to FIG. 1, remote PHY node 108 contains a chipset unit122 (ASIC or FPGA), a forward amplifier 124, a return amplifier 126, adiplex filter 128, and a tap 130 containing an output test port 131.Chipset unit 122 contains chipsets that include downstream QAMmodulators, upstream QAM demodulators, and logic to connect andcommunicate with CCAP core 104. A downstream or forward signal producedin chipset unit 122 is amplified by forward amp 124, passes through ahighpass filter leg of diplex filter 128, passes through tap 130, exitsnode 108 at an RF output port, and then travels to coaxial cable plant110. Upstream or return signals from subscriber networks 112 travel upthrough plant 110 and are received by node 108. The return signals passthrough a lowpass filter leg of diplex filter 128, are amplified byreturn amp 126, received by chipset unit 122 for demodulation anddown-conversion, and then communicated to CCAP core 104 via digitalfiber optic cable 106. The general purpose of test port 131 is tomeasure and calibrate the level of the forward signal. Chipset unit 122may include model BCM3047 downstream modulator chipset and modelBCM31442 advanced DOCSIS® 3.1 cable network burst receiver chipset, bothmanufactured by Broadcom Inc., San Jose, Calif. (“Broadcom”).

Again referring to FIG. 1, a CPD generator 132, constructed inaccordance with the present invention, is coupled to remote PHY node 108at test port 131. Port 131 is an output test port having an intrinsicattenuation of 20 dB; thus, a sampling of the forward signal from node108 is available at port 131, reduced in level by 20 dB. CPD generator132 is configured to receive and receives the forward signal from port131. Using the forward signal, generator 132 generates a CPD signal thatsimulates an actual CPD signal generated in cable plant 110. CPDgenerator 132 also, according to the present invention, limits the bandof the simulated CPD signal to a particular return channel used byremote PHY node 108, cable plant 110, and subscriber networks 112. Thisband-limited, simulated CPD signal is referred to herein as a “referenceCPD signal”—reference CPD signal 133 in FIG. 1. As shown in FIG. 1, CPDgenerator 132 supplies reference CPD signal 133 to node 108 via port131, tap 130, the lowpass filter leg of diplex filter 128, return amp126, and to chipset unit 122. Chipset unit 122 receives reference CPDsignal 133 in the return channel. Preferably, the return channel is anindustry standard out-of-band (OOB) return channel, and more preferably,the return channel is a narrowband digital return (NDR) channel definedunder DOCSIS specifications. Further details of CPD generator 132 andits operation are provided below.

Again with respect to FIG. 1, the forward signal is delivered to coaxialcable plant 110 from remote PHY node 108. An actual CPD signal (or CPDecho or CPD echo signal) 134 is generated in cable plant 110 by aninteraction between of the forward signal and a CPD source 136 (theso-called “diode effect;” see Background). Because of the nonlinearityof CPD source 136, intermodulation products of the forward signal aregenerated at source 136, and the lower frequency intermodulation (IM)products travel upstream (as shown by dotted-line arrow). These lowerfrequency IM products collectively constitute the actual CPD (echo)signal 134, which has frequencies extending across the wholeupstream/return spectrum (including the return channel, e.g., OOB returnchannel). Thus, CPD signal 134 can potentially raise the noise floor andinterfere with normal upstream service signals or traffic. CPD signal134 travels through bi-directional amplifier 138 and is ultimatelyreceived by remote PHY node 108. Signal 134 continues through tap 130,the lowpass filter leg of diplex filter 128, return amp 126, and tochipset unit 122. Chipset unit 122 captures the frequency components ofCPD signal 134 that are within the return (OOB) channel, for CPDdetection and location processing. For the purposes of CPD detection andlocation, the portion of actual CPD signal 134 within the return channelis simply referred to as the actual CPD signal 134. Note that referenceCPD signal 133 (from CPD generator 132) and actual CPD signal 134 areboth received or captured by chipset unit 122 within the return channel(the same frequency channel).

The forward signal propagates from remote PHY node 108 to CPD source 136and CPD signal 134 propagates from CPD source 136 to remote PHY node108, all within a round-trip propagation interval (or “round-tripinterval”). Synchronous capture of the forward signal and CPD signal 134is defined as the capture of both signals occurring within theround-trip interval. The forward signal is captured and reference CPDsignal 133 is generated and supplied to remote PHY node 108 (chipsetunit 122) at the beginning of the round-trip interval. Actual CPD signal134 is captured by remote PHY node 108 (chipset unit 122) at the end ofthe round-trip interval. The time relationship (or synchronization)between reference CPD signal 133 and actual CPD signal 134 is preserved,because both signals are received (or captured) in chipset unit 122 inthe same return channel. Thus, timestamps for each signal are notrequired to preserve synchronization between the two signals. The terms“capture” and “captured” mean receive and received, respectively. Theseterms are used, for the most part, when specifically referring tosynchronous capture of the forward signal and actual CPD signal 134.

Chipset unit 122 is configured to receive signals of the return channel.Chipset unit 122 includes digital in-phase (I) and quadrature (Q)down-converters for converting signals received from the return channelto I and Q baseband signal samples. These I/Q baseband samples are thentransmitted or streamed to CCAP core 104 over fiber cable 106, in theform of Ethernet or PON digital (IP) signals. An aspect and advantage ofthe preferred embodiments of the present invention is to utilize anexisting return channel for which chipset unit 122 is designed, toreceive certain signals from subscriber networks 112. Thus, chipset unit122 (or any part of node 108) does not have to be specially configuredto carryout the present invention. As indicated earlier, reference andactual CPD signals 133, 134 are received or captured by chipset unit 122in the return channel. At chipset unit 122, reference CPD signal 133will have a zero time delay and actual CPD signal 134 will have arelative time delay of about the round-trip interval. Thesesignals—treated as a combined return signal—are down-converted to I/Qbaseband signal samples and transmitted to CCAP 104 via fiber opticcable 106. At the request of CPD core 118, or in accordance with apre-programmed routine in CPD core 118 and/or CCAP core 104, the I/Qsamples of reference and actual CPD signals 133, 134 (combined returnsignal) are sent from CCAP core 104 to CPD core 118 for CPD detectionand location processing. The I/Q samples include the components of bothsignals and are processed by core 118 as a combined return signal.

The types of return channels that can be utilized by the preferredembodiment of the present invention are described in Data-Over-CableService Interface Specifications, DCAM-HAv2, Remote Out-of-BandSpecification, CM-SP-R-OOB-I06-170524 (May 24, 2017), published byCableLabs®, which is incorporated herein by reference (hereafter “OOBSpecification”). These remote PHY return channels are also referred toas out-of-band (OOB) return channels. These channels are used to supportphysical layer (PHY) signaling for set-top box (STBs) equipment insubscriber networks 112. Millions of deployed STBs are dependent uponOOB return channels for localization, video control/enablement datadelivery, code upgrades, and two-way interactive applications. Theheadend implementation for supporting remote PHY OOB signals isgenerally found in the CCAP core, but may also be in standalonefunction-dedicated auxiliary cores. A general description of remote PHYOOB signal channels are given in Section 5 (Overview) of the OOBSpecification. The different return channel types include: (1) the SCTE55-2 Remote PHY solution; (2) the SCTE 55-1 Remote PHY solution; (3) thenarrowband digital return (NDR); (4) pilots; and (5) CW tones. Note thatthe present invention may also be implemented with an alternative returnsignaling mechanism using DOCSIS as a transport, called DOCSIS Set-topGateway or DSG. See Data-Over-Cable Service Interface Specifications,DOCSIS Set-top Gateway (DSG) Interface Specification,CM-SP-DSG-I24-130808 (Aug. 8, 2013), published by CableLabs®, which isincorporated herein by reference. The term “return channel,” indescribing and claiming the present invention, shall be interpreted toinclude, but not be limited to, all the above mentioned return channeltypes and alternatives.

NDR is the most preferred return channel for the purposes of the presentinvention. An NDR channel sends digital samples as payload withinpackets which are transmitted or streamed from the remote PHY device tothe CCAP core (or CMTS core). NDR can work with any type of OOB signalas long as the signal is contained within the defined passband (e.g.,9-14 MHz). The NDR channel is described in greater detail in Section 7.2of the OOB Specification. The deployment of NDR is a “Must” requirementunder the OOB specification; thus, the communication requirements of thepresent invention are or will be in-place in most modern HFC networksystems. I/Q samples of reference and actual CPD signals 133, 134 can bereceived from CCAP core 104 by using standard communication protocols byany core, including CPD core 104. In other words, practicalimplementations of the present invention do not require additionalsoftware at CCAP core 104 or remote PHY device 108. The use of the NDRchannel is also attractive because the NDR channel has a bandwidth (orpassband) of 5.12 MHz, which provides reasonable time-distanceresolution (around 0.2 μs or 30 meters) and good accuracy of ranging aCPD source in coaxial plant 110 (around 3 meters). Another advantage ofusing the NDR channel is that for transmission of baseband FQ samples,only a minimal bit rate is required, which allows for long accumulationtimes (processing gain) at a correlation detector (e.g., in CPD core118) without processing relatively large amounts of captured data. Forexample, an accumulation time of 100 ms (as in ARCOM Hunter® Xcor®System) will produce only about 1.3 MB of data.

In CPD generator 132, the simulated CPD signal is band-limited to theNDR channel to generate reference CPD signal 133 (i.e., reference CPDsignal 133 has a bandwidth across the NDR band). The signal level ofreference CPD signal 133 should be about 10-20 dB above noise in the NDRchannel at chipset unit 122. This level will be sufficient to generatesuitable samples of reference CPD signal 133 for correlation detectionat CPD core 118. By limiting the level of reference CPD signal 133 to10-20 dB above noise in the NDR channel, the same NDR channel can beused simultaneously for receiving other OOB signals, such as FSK signalsfrom STBs, etc. Thus, the original utility or use of the NDR channelshould not be affected by most embodiments of the present invention.

Referring now to FIG. 2, a block diagram of a passive, analog CPDgenerator 200, of the present invention, is shown. CPD generator 200includes a coupling element 202, a diplex filter 204, a CPD simulatorcircuit 206, and a bandpass filter 208. CPD generator 200 is coupled toa remote PHY node 210 via coupling element 202, a test port 213, and anoutput tap 212. Output tap 212 is associated with remote PHY node 210.Tap 212 may be separate from or integral with node 210. If separate, tap212 is located very close to the output of node 210. In either case, tap212 is considered associated with node 210. A forward signal produced bynode 210 travels downstream to a coaxial cable plant 214. A sampling ofthe forward signal is tapped from tap 212 via test port 213. Diplexfilter 204 has high and low frequency paths (e.g., highpass and lowpassfilters). The tapped forward signal is received by CPD generator 200through coupling element 202 and the high frequency path of diplexcircuit 204. The forward signal is then received by CPD simulatorcircuit 206 (e.g., a Schottky diode), which generates IM products of theforward signal. A low frequency band of the IM products is selected bybandpass filter 208, which has a passband substantially equal to the OOBreturn channel used in node 210. In this example, the return channel is9 to 14 MHz. The band-limited IM products from filter 208 constitute thereference CPD signal, which is allowed to pass through the low frequencypath of diplex circuit 204 and exit out coupling element 202. Thereference CPD signal then travels through port 213, tap 212, and intoremote PHY node 210, as described earlier with reference to FIG. 1.

As shown in FIG. 2, an actual CPD echo signal (created at a CPD sourcein cable plant 214 from the forward signal) travels back from cableplant 214 to node 210. Both reference and actual CPD signals arereceived by remote PHY node 210 in the return channel, within theround-trip interval (as defined earlier). Typically, the reference CPDsignal is received by node 210 at the beginning of the round-tripinterval and the actual CPD echo signal is captured by node 210 at theend of the round-trip interval. Thus, the reference and actual CPDsignals are synchronized, in that the relative time delay between theirarrivals in node 210 is maintained within the return channel (e.g., NDRchannel).

FIG. 3 shows a frequency plot of a return path spectrum 300 as viewed byremote PHY node 210 (FIG. 2). The plot extends from 2 to 50 MHz. Asindicated, a reference CPD signal 302 and an actual CPD echo signal 304are both located within a return channel at 9-14 MHz. This channel isthe NDR channel. In this example, reference CPD signal 302 is producedby CPD generator 200 (FIG. 2) and actual CPD signal 304 is produced by aCPD source in cable plant 214. As shown in FIG. 3, the stronger spectralcomponent in the NDR channel belongs to reference CPD signal 302 and theweaker spectral component (at or below noise level) belongs to actualCPD echo signal 304. Signals 302 and 304 are sync captured and will beprocessed as one combined return signal by a chipset in node 210.

Referring now to FIG. 4, there is shown a block diagram of a morespecific embodiment of the present invention. A passive, analog CPDgenerator 400 comprises a coupling element 402, a highpass filter 404,an attenuator 406, a CPD simulator circuit 408, and a bandpass filter410. Coupling element 402 may be any suitable type of RF signal couplingdevice or devices such as an RF cable connector (e.g., an F-connector),a coaxial jumper cable, a three-port coupler (e.g., in activeembodiments), a combination of these, etc. Highpass filter 404 ispreferably a passive LC filter (see FIG. 6) with a passband preferablyextending over the entire forward path spectrum (e.g., 54 to 1002 MHz).Attenuator 406 is preferably a resistive Pi network (FIG. 6) whichproduces preferably 10 dB of attenuation. CPD simulator circuit 408 ispreferably a Schottky diode (FIG. 6). Attenuator 406 is employed tofurther reduce the level of the forward signal and to match or isolatethe connection between highpass filter 404 and CPD simulator circuit 408(the Schottky diode). Bandpass filter 410 is preferably a passive LCfilter (FIG. 6) with a passband preferably limited to and as wide as theNDR channel (e.g., 9-14 MHz). Optionally, a diplex filter may beinserted between coupling element 402 and filters 404 and 410, toaddress any impedance matching and isolation concerns.

FIGS. 5A through 5D are a series of frequency sweeps of a forward signalbeing processed at different points within CPD generator 400 (FIG. 4).FIG. 5A displays the forward signal just after it has been filtered byhighpass filter 404. FIG. 5B displays the forward signal after it hasbeen attenuated by attenuator 406 and has interacted with CPD simulatorcircuit 408 (Schottky diode). Compare the signals displayed in FIGS. 5Aand 5B. Because circuit 408 (Schottky diode) is a non-linear element,the signal in FIG. 5B is distorted by IM products. High and lowfrequency IM products 502, 504 can clearly be seen above and below theforward path spectrum. IM products 504 (in the return spectrum) have thesame structure as an actual CPD echo signal (e.g., signal 134 in FIG.1). That is why IM products 504 are used as a reference signal fordetection of CPD by a cross-correlation method. FIG. 5C displays thereturn channel portion of the signal of FIG. 5B, where the signal ofFIG. 5B has been filtered by bandpass filter 410. Note, the forwardsignal frequencies have been rejected or suppressed, as indicated by506. Low frequency IM products 504 within the passband of filter 410 areshown in FIG. 5C as a reference CPD signal 508 (at 9-14 MHz). Lastly,FIG. 5D displays the band-limited signal 508 at a different scale, as anoutput signal 510 (reference CPD signal) of CPD generator 400. Signal510 extends over the NDR channel with a bandwidth of about 5 MHz(actually 5.12 MHz) and centered at 11.5 MHz.

Referring now to FIG. 6, there is shown a circuit 600 of passive, analogCPD generator 400 (FIG. 4). A coaxial connector 602, corresponding tocoupling element 402, functions as an input (for the forward signal) andan output (for the reference CPD signal). On the lower leg of circuit600, there is a passive highpass filter 604 (corresponding to filter 404in FIG. 4) realized with inductors L8-L10 and capacitors C8-C11, whichproduce the output shown in FIG. 5A. Also on the lower leg, anattenuator 606 (corresponding to attenuator 406) is realized with a Piarrangement of resistors R1-R3 having values that produce 10 dB ofattenuation. A Schottky diode 608 serves as the CPD simulator circuit(corresponding to circuit 408) and produces the response shown in FIG.5B. On the upper leg of circuit 600, there is a bandpass filter 610(corresponding to filter 410 in FIG. 4) realized with inductors L1-L7and capacitors C1-C7, which produce the responses shown in FIGS. 5C &5D. The component values for the inductors, capacitors and resistorsshown in FIG. 6 are representative of a practical design; however, theinvention (and even this embodiment) is not limited to these values.

The signal levels in circuit 600 will now be discussed with reference toFIGS. 1 and 6. A typical level of a forward signal from test port 131(at connector 602) is about 30 dBmV (for 6 MHz QAM channel). If thevalue of attenuation of attenuator 606 is 10 dB, than the forward signallevel at Schottky diode 608 will be about 20 dBmV. At diode 608, theforward signal generates IM products that are about −30 dB below theforward level; thus, the level of the IM products after diode 608 andbandpass filter 610 will be about −10 dBmV. Thus, the output of CPDgenerator 600 (at connector 602), i.e., the reference CPD signal, willbe about −10 dBmV. This level is reduced by 20 dB after the referenceCPD signal passes through test port 131 (FIG. 1). Thus, the referenceCPD signal level is −30 dBmV after passing through tap 130 and thelowpass filter leg of diplex filter 128 (FIG. 1). Depending on the gainof return amp 126, the level of the reference CPD signal can be adjustedby selecting the attenuation of attenuator 606 to provide a suitablesignal level at the input of chipset unit 112 (FIG. 1).

As indicated above, the expected signal level of IM products from diode608 is about −10 dBmV. The high frequency IM products (i.e., IM products502 in FIG. 5B) will travel in a reverse path through attenuator 606(−10 dB) and highpass filter 604, and will have a level of −20 dBmV atconnector 602. The isolation of tap 130 for a signal entering port 131and leaking downstream of tap 130, is about 35-45 dB. Thus, the nominallevel of the high frequency IM products (502 in FIG. 5B) traveling fromconnector 602, through tap 130, and to coaxial cable plant 110, will beabout −55 to −65 dBmV. A typical level of the forward signal travelingfrom remote PHY node 108 to cable plant 110 is about +50 dBmV. Thismeans that the high frequency IM signal (502 in FIG. 5B) will be atleast 100 dB below the forward signal level at cable plant 110. Thus,the high frequency IM products leaking from CPD generator circuit 600will not interfere with the forward signal in plant 110.

Referring now to FIG. 7A, a passive, analog CPD generator 700 a includesa trap or cylindrical, hermetically-sealed, metal housing 704 a. CPDGenerator 700 a is intended to be installed on the outside of a remotePHY device 701 a. CPD generator housing 704 a includes an RF coaxialcable connector 702 a, such as an F-connector. Contained within housing704 a is a circuit board 706 a populated with circuit 600 (FIG. 6).Connector 702 a is configured to connect physically and electrically toa corresponding, externally disposed connector 703 a of remote PHYdevice 701 a. CPD generator housing 704 a is shown with its cylindricalwall cut-away to show circuit board 706 a mounted therein. Theconstruction of housing 704 a and the assembly of circuit board 706 awithin housing 704 a are well-known and will not be further described.An example of a similar housing and board construction/assembly isdisclosed in U.S. Pat. No. 8,458,759 (Jun. 4, 2013) to Zinevich et al.,which is incorporated herein by reference. An example of remote PHYdevice 701 a is a product by Harmonic Inc., 4300 North First Street, SanJose, Calif., called CableOS Ripple-1(www.harmonicinc.com/products/product-detail/remote-phy-node-cableos-ripple1).

Referring to FIG. 7B, a passive, analog CPD generator 700 b includes acircuit board 706 b populated with circuit 600 (FIG. 6). Optionally,board 706 b may be enclosed in a protective enclosure, jacket or shield.CPD Generator 700 b is intended to be installed within a housing of aremote PHY device 701 b. An RF coaxial cable connector 702 b (e.g., anF-connector) is rigidly mounted to circuit board 706 b. Connector 702 bis configured to connect physically and electrically to a corresponding,internally disposed connector 703 b of remote PHY device 701 b.

Referring back to FIG. 1, the I/Q baseband samples of the return channel(e.g., NDR channel), produced by chipset unit 122, are received by CCAPcore 104 via optical fiber cable plant 106. CCAP 104 relays the I/Qbaseband samples to CPD core 118, either automatically or at the requestof CPD core 118. CPD core 118 captures the I/Q baseband samples during atime interval long enough to provide sufficient processing gain in acorrelation detector implemented within CPD core 118. This time intervalmay be referred to as accumulation time. The sensitivity of detection ofCPD is dependent on the processing gain or accumulation time. As will beexplained below, for most embodiments, the accumulation time should beabout 800 milliseconds to achieve a CPD detection sensitivity of 30 dBbelow the system/channel noise floor.

The combined return signal S (both reference and actual CPD signals)received by CPD core 118 can be represented as:S=Sr+Sa+N,

-   -   where:    -   Sr is reference CPD signal 133 of CPD simulator 132;    -   Sa is actual CPD echo signal 134 from CPD source 136;    -   N is ingress from cable plant 110 plus internal noise in the NDR        channel        In the preferred embodiment, as CPD core 118 receives the        combined return signal S, CPD core 118 begins to calculate the        autocorrelation function A(t) of signal S, which can be        represented as:        A(t)=A1(t)+A2(t)+C(t),    -   where:    -   A1(t)=(Sr×Sr), the autocorrelation function of reference CPD        signal 133;    -   A2(t)=(Sa+N)×(Sa+N), the autocorrelation function of actual CPD        signal 134 plus ingress/noise N;    -   C(t)=(Sr×(Sa+N)), the cross-correlation function of reference        and actual CPD signals plus ingress/noise signal N.        Cross-correlation function C(t) corresponds to the response of        an optimal cross-correlation detector (or a matched filter        configured to match to actual CPD signal 134). Each correlation        peak of cross-correlation function C(t) has a time delay        corresponding to an associated round-trip interval (i.e., time        of forward signal to propagate from remote PHY node 108 to the        CPD source plus time of the actual CPD signal to propagate from        the CPD source back to remote PHY node 108). In accordance with        the preferred embodiment of the present invention, CPD detection        includes extraction and detection of peaks of cross-correlation        function C(t) from the autocorrelation function A(t). The term        “extraction,” in the context of the present invention, means        removing or minimizing the masking effect (or correlation noise)        of at least the autocorrelation function A1(t) on the        cross-correlation peaks of function C(t).

FIG. 8 shows an envelope 800 of an autocorrelation function A(t) of acombined return signal. Autocorrelation A(t) is referred to herein as acombined autocorrelation function. Envelope 800 was computed from datacollected in a bench test of the present invention, where the returnchannel is the NDR channel having a bandwidth of 5.12 MHz. Envelope 800is a plot of signal level in dBmV versus time delay in microseconds. Asshown, envelope 800 has a first peak 802 at zero time delay, a secondpeak 804 corresponding to a first actual CPD echo signal with a 1.5 μstime delay, a third peak 806 corresponding to a second actual CPD echosignal with a 8 μs time delay, and a “correlation noise” floor 808formed primarily by sidelobes of composite function A(t). Floor 808 islike a real noise floor in that it limits the sensitivity of detectionof a CPD cross-correlation peak. It is sometimes referred to simply as a“noise floor’ below.

FIG. 9 is a diagram of the structure of the combined autocorrelationfunction A(t) of FIG. 8. The structure delineates the contributions ofthe different terms of composite function A(t), including anautocorrelation function of the reference CPD signal A1(t), theautocorrelation function of the actual CPD (echo) signal plus noiseA2(t), and the cross-correlation function C(t) of the reference andactual CPD signals. The composite function A(t) and the different termsof A(t) are plotted in FIG. 9 in units of signal level versus timedelay. Composite function A(t) has a first peak 902 at zero time delay,a second peak 904 at a time delay t1, a third peak 906 at a time delayt2, a fourth peak 905 at a time delay of t2−t1, and a correlation noisefloor 908. Autocorrelation term A1(t) has a peak 902 a and a correlationnoise floor 908 a. Autocorrelation term A2(t) has a peak 902 b, a peak905 b, and a correlation noise floor 908 b. Cross-correlation term C(t)has a peak 904 c and a peak 906 c. Peaks 904 c and 906 c representdetection of two actual CPD echo signals originating from differentlylocated CPD sources (thus, the peaks are at two different time delays,t1 and t2).

In order to produce uncorrupted or suitable reference CPD signal samplesfor CPD detection, a relatively high signal level (relative to actualCPD echo signal) for the reference CPD signal is desired. As mentionedearlier, the reference CPD signal may have a level at the output of CPDgenerator 132 (or 400 or 600) of about −10 dBmV, which also may be thelevel at chipset unit 122 assuming return amp 126 has 20 dB gain). Thus,the autocorrelation function of the reference CPD signal, A1(t), will bethe strongest contributor in the combined autocorrelation function A(t).This is illustrated in FIG. 9, where function A1(t) has the highest peak(peak 902 a) and the highest correlation noise floor (floor 908 a) ofall the terms, A1(t), A2(t) and C(t). Peak 902 of composite functionA(t) masks cross-correlation peak 904 c, which represents a CPD sourceclose to remote PHY node 108. Correlation noise floor 908 of compositefunction A(t) masks cross-correlation peaks 904 c and 906 c, where noisefloor 908 is mainly caused by sidelobes (noise floor) of function A1(t).This result is counter to optimal CPD detection, which utilizes thecross-correlation function C(t). Thus, it would be desirable to removeor at least minimize the masking or shadowing effect of A1(t) oncross-correlation peaks 904 c and 906 c (a process called “extraction”).

Other features in FIG. 9 are now discussed. Peak 902 b ofautocorrelation function A2(t), at zero time delay, contributes toadditional masking of cross-correlation peak 904 c. Also, function A2(t)has peak 905 b, which results from the cross-correlation between the twoactual CPD echo signals (represented by peaks 904 c & 906 c). Peak 905 bhas a time delay T=t2−t1. Peak 905 b is referred to as a “false peak.” Afalse peak may also interfere with the detection of cross-correlationpeaks 904 c and 906 c. However, in general, cross-correlation peaks 904c and 906 c will have a higher level than a false peak. In addition,technicians are typically trained to remove first a CPD sourcerepresented by the highest cross-correlation peak. If this routine isfollowed, then the false peak will simply disappear upon removal of thefirst CPD source. As established above, function A1(t) is the primaryinterference contributor to CPD detection (sometimes referred to as“correlation interference”).

Correlation noise floor 908 in function A(t) can be reduced byincreasing the duration of the captured combined returned signal. Thisis equivalent to increasing the accumulation time or processing gain forthe combined autocorrelation. This step is illustrated in FIGS. 10A and10B. FIG. 10A shows an autocorrelation function 1000 a of a combinedreturn signal in an NDR channel of 5.12 MHz, obtained from datacollected in an actual bench test. Function 1000 a is very similar tothe function in FIG. 8. Both represent the same test setup, with two CPDsources generating a cross-correlation peak at about 1.5 μs and anotherat about 8 μs. However, in FIG. 10A, function 1000 a is based on acombined return signal having a duration (or accumulation time) of only50 milliseconds. Notice a correlation noise floor 1002 a at a level ofabout −65 dBmV. Now compare this with a combined autocorrelationfunction 1000 b in FIG. 10B. FIG. 10B represents the same test setup asFIG. 10A. However, function 1000 b is based on a combined return signalhaving a duration (or accumulation time) of 800 milliseconds. Notice acorrelation noise floor 1002 b at a level of about −80 dBmV. Thus, thenoise floor has been reduced by more than 12 dB (≈15 dB). FIGS. 10A and10B demonstrate a reduction in noise floor by increasing the effectiveaccumulation time of the autocorrelation. The reduced correlation noisefloor in FIG. 10B means that the cross-correlation (CPD) peaks can bedetected with increased sensitivity of about 12-15 dB. Thus, aneffective accumulation time of 800 milliseconds is a recommendedparameter for the autocorrelation operation in most embodiments of thepresent invention.

In addition to the above step, or as an alternative, a cancellationmethod 1100, outlined in FIG. 11, may be employed to reduce the maskingeffect of main peak 902 (FIG. 9) and its sidelobes. In a first step 1102of method 1100, a reference CPD signal is generated from a forwardsignal and I/Q baseband data samples are created. This step may becarried out in the field, where, for example, the output of tap 130(FIG. 1) is terminated while CPD generator 132 generates a reference CPDsignal from the forward signal captured from port 131. Then, thereference CPD signal is supplied to remote PHY node 108 and received inchipset unit 122. Baseband FQ samples of the reference CPD signal arecreated in chipset unit 122 and then sent to CCAP core 104 and then toCPD core 118. In a second step 1104, an autocorrelation function A1(t)of the reference CPD signal data is computed. The computed A1(t) isstored in memory in CPD core 118 as a “calibration curve.” As apractical matter, the field approach would take place upon installationof remote PHY node 108. A calibration curve from one field installationof node 108 could be saved and distributed as a default curve for allother remote PHY nodes in the field. In an alternative approach, steps1102 and 1104 are carried out in a lab or factory as part of a benchcalibration procedure. Note, it is preferred that the computation ofA1(t) include the complete complex autocorrelation function for A1(t).

The description of method 1100 continues. In a third step 1106, thenormal operation of CPD generator 132 and remote PHY node 108 isperformed to generate a combined return signal (I/Q baseband datasamples) from reference and actual CPD signals captured in the returnchannel in remote PHY node 108. The combined return signal data is sentto the CCAP core and then to the CPD core. In a fourth step 1108, anautocorrelation function A(t) of the combined return signal data iscomputed in CPD core 118. In a fifth step 1110, the stored calibrationcurve or function A1(t) is subtracted from autocorrelation functionA(t). This step will substantially cancel at least the main peak (e.g.,902) at zero time delay, associated with the reference CPD signal. Themain peak will be canceled by more than 40-50 dB (due to the strength ofthe A1(t) term). The cancellation of peak 902 will substantially unmaskthe cross-correlation peak closest to zero (e.g., 904 c), which isassociated with the actual CPD echo signal. The correlation noise floor(or sidelobes of peak 902) may be reduced also in this step. In a sixthstep 1112, the unmasked cross-correlation peak is detected in CPD core118. A further step in the method may be to update the calibration curvefrom time-to-time by repeating steps 1102 and 1104.

FIG. 12 shows a plot 1200 (in linear amplitude scale) of the complexautocorrelation function A1(t) computed and stored in step 1102 and usedin step 1106 to subtract from the combined autocorrelation functionA(t). The complex function includes a real part 1202 and an imaginarypart 1204. Both parts are subtracted from corresponding parts of thecombined autocorrelation function produced in step 1104 (FIG. 11).

FIGS. 13A and 13B are before-and-after results, derived from a benchtest, that demonstrate the effectiveness of cancellation method 1100.FIG. 13A shows a combined autocorrelation function A(t), as obtained instep 1104, before subtraction of the calibration curve obtained/storedin step 1102. Note the presence of a main peak 1302 and a nearby maskedcross-correlation peak 1304. FIG. 13B shows a corrected autocorrelationfunction A′(t), which is the result after subtraction of the calibrationcurve in step 1106. Note from an inspection of A′(t), main peak 1302 issubstantially removed. Remnants of the peak, designated as 1302′ in FIG.13B, are more than 50 dB down from the maximum level of original peak1302. Also, nearby cross-correlation peak 1304 is shown in FIG. 13B asbeing substantially unmasked and cleared for detection (with asignal-to-noise ratio of about 15 dB).

It should be noted that method 1100 (FIG. 11) will work well with lowlevel actual CPD echo signals and ingress from the coaxial cable plant.However, with an increase in levels of actual CPD echo signals andingress, the efficiency of cancellation will be reduced, because thecontribution of peak 902 b (see FIG. 9) of function A2(t) will beincreased. On the other hand, in the case of increased levels of actualCPD echo signals, the levels of the cross-correlation (CPD) peaks (e.g.,904 c & 906 c in FIG. 9) will also increase. Thus, the negative effectof increased CPD echo signal levels will be offset by the increasedcross-correlation peak levels. More of a concern is an increase iningress level in the coaxial cable plant. However, ingress level usuallyvaries over the course of a day, so CPD detection can be carried outaccording to a scheduler and when the ingress level is minimal. Also, inmany cases, ingress behaves like pulse noise. In such cases, ingresspulses, received as part of the FQ baseband samples, could be blanked(e.g., by zero stuffing) in CPD core 118 before the autocorrelationcomputation.

Heretofore, a passive CPD generator embodiment has been discussed. Apassive solution is preferable from the point of view of simplicity,cost, and compatibility with different models of remote PHY devices.However, active embodiments of the present invention are alsocontemplated. Active embodiments are those requiring power to operate.As will be discussed, the active elements are signal processingcomponents. These components are used to overcome interference betweenthe reference and the actual CPD signals and/or to better resolve thetwo signals, for improved CPD detection and location. The activeembodiments are discussed below with reference to FIGS. 14-17.

Referring now to FIG. 14, there is shown a block diagram of a CPDgenerator 1400, constructed in accordance with a second embodiment ofthe present invention. CPD generator 1400 is an active device, requiringpower. Generator 1400 includes a coupler element 1402, a diplex filter1404, a CPD simulator circuit 1406, a bandpass filter 1408, a pulsemodulator 1410, a digital delay line 1412, a controller 1414, and apower input port 1416. Coupler element 1402 is preferably an RFF-connector for connecting directly to a test port associated with aremote PHY device. In operation, a forward signal from a remote PHYdevice is received by generator 1400 via connector 1402. Diplex filter1404 has high and low frequency paths (e.g., highpass and lowpassfilters). The forward signal is received through the high frequency pathof diplex filter 1404. CPD simulator circuit 1406 is preferably aSchottky diode, but, in this active embodiment, it may be a FETtransistor circuit. CPD simulator 1406 generates IM products of theforward signal, and a low frequency band of the IM products is selectedby bandpass filter 1408. Filter 1408 has a passband substantially equalto the OOB return channel (e.g., NDR channel) used in the associatedremote PHY device. The band-limited output of filter 1408 constitutesthe reference CPD signal.

Preferably, pulse modulator 1410, delay line 1412, and controller 1414are all implemented in a field-programmable gate array (FPGA) chip 1409.One suitable FPGA chip is a Xilinx® FPGA XC3S400A-4FTG256C, manufacturedXilinx, Inc., San Jose, Calif. Pulse modulator 1410 may be implementedin the FPGA as an ON/OFF switch of digital samples. Pulse modulator 1410and delay line 1412 may instead be implemented as separate commerciallyavailable microchips. Controller 1414 controls the operation of pulsemodulator 1410 and delay line 1412. In operation, the reference CPDsignal is processed by pulse modulator 1410 to shape the reference CPDsignal into a periodic sequence of pulses. FIG. 15 shows the referenceCPD signal in the form of pulses 1502 a, 1502 b, 1502 c, . . . (topdiagram). Pulses 1502 a, 1502 b, 1502 c, . . . are then delayed by aunit of time, delta t, in digital delay line 1412. The output of delayline 1412 is a sequence of delayed pulses 1502 a′, 1502 b′, 1502 c′, . .. (bottom diagram in FIG. 15). The delayed pulses pass through the lowfrequency path of diplex filter 1404 and then through connector 1402 tothe remote PHY device. Delta t is chosen to be greater than the expectedmaximum round-trip interval (between the remote PHY device and a CPDsource in the coaxial cable plant). As an example, delta t should begreater than 50 μs. This will allow the actual CPD echo signal to bereceived in the remote PHY device without any interference by thereference CPD signal (in the same channel). In other words, the CPD echosignal is received during a “quiet period,” during which the referenceCPD signal is being delayed (see bottom diagram in FIG. 15). Thisapproach is akin to the simplex method of communication, where the samechannel is used for both transmission and reception sessions.

Power for the active circuits in CPD generator 1400 is obtained from theassociated remote PHY device via a power output port on the device. Thepower from the remote PHY device is usually +24 VDC. A power cable isconnected between the output port of the remote PHY device and powerinput port 1416. In some remote PHY devices, a combined RF and poweroutput port may be available to permit power to be received throughcoupler element 1402. In CPD generator 1400, a power supply (not shown)will divide the voltage down to appropriate values for powering the FPGAchip or any other microchips used to implement pulse modulator 1410,digital delay line 1412, and/or controller 1414.

In the second embodiment, the accumulation time of the autocorrelationshould also be 800 milliseconds. If the pulse width of each pulse 1502a, 1502 b, 1502 c, . . . is 100 milliseconds and delta t is 100milliseconds, then four pulses and four delta t intervals will cover 800milliseconds for the composite autocorrelation function. In thisembodiment, the reference CPD signal level should be set higher than inthe passive embodiment, because of the reduced risk of interference withthe actual CPD echo signal and the desire to obtain high fidelityreference samples.

Referring now to FIG. 16, there is shown a block diagram of a CPDgenerator 1600, constructed in accordance with a third embodiment of thepresent invention. CPD generator 1600 is an active device, requiringpower. Generator 1600 includes a coupler element 1602, a diplex filter1604, a CPD simulator circuit 1606, a bandpass filter 1608, a pulsemodulator 1610, a phase inverter circuit 1612, an RF switch 1613, acontroller 1614, and a power input port 1616. Coupler element 1602 ispreferably an RF F-connector for connecting directly to a test portassociated with a remote PHY device. Generator 1600 receives a forwardsignal from the remote PHY device via connector 1602. Diplex filter 1604has high and low frequency paths, and the forward signal is receivedthrough the high frequency path. CPD simulator circuit 1606 ispreferably a Schottky diode, but may be a FET transistor circuit. CPDsimulator 1606 generates IM products of the forward signal. A lowfrequency band of the IM products is selected by bandpass filter 1608.Filter 1608 has a passband substantially equal to the OOB return channel(e.g., NDR channel) used in the associated remote PHY device. Theband-limited output of filter 1608 constitutes the reference CPD signal.

In this third embodiment, pulse modulator 1610 and RF switch 1613 may becommercially available microchips, and phase inverter circuit 1612 maybe an analog transformer or a transistor circuit. Preferably, controller1614 is implemented in a FPGA chip, as in the second embodiment.Controller 1614 controls the operation of pulse modulator 1610 and RFswitch 1613. In operation, the reference CPD signal is processed bypulse modulator 1610 to shape the reference CPD signal into a sequenceof long-duration pulses. Then, every other pulse of the sequence isphase inverted (i.e., shifted 180 degrees relative to the un-invertedpulses). See the sequence of pulses 1700 in FIG. 17. Phase-inversion ofevery other pulse is accomplished by phase inverter 1612 in conjunctionwith RF switch 1613 and under control of controller 1614. Each of thepulses 1700 (0/180 degrees) represents a reference CPD signal,preferably about 800 milliseconds in duration. Pulses 1700 pass throughthe low frequency path of diplex filter 1604 and then through connector1602 to the remote PHY device. Referring to FIG. 17, a first pulse 1702has a phase of zero degrees and a second subsequent pulse 1704 has aphase of 180 degrees. Pulse 1702, representing the reference CPD signal,is received by the remote PHY device from generator 1600. The remote PHYdevice also receives an associated actual CPD echo signal from thecoaxial cable plant. As in the passive embodiment, both signals areprocessed as a combined return signal. At a CPD core, a combinedautocorrelation function 1706 (FIG. 17) is computed from the combinedreturn signal. This process is repeated for second pulse 1704. Pulse1704 is received by the remote PHY device from generator 1600, andremote PHY device also receives a second associated actual CPD echosignal from the coaxial plant. Both signals are processed as a combinedreturn signal, and an autocorrelation function 1708 (FIG. 17) isgenerated therefrom.

Because autocorrelation function 1708 was computed from a combinedsignal having a 180 degree phase-shifted reference signal, thecross-correlation terms of the function will be negative (−C(t)). Thus,in the next processing step 1710 (FIG. 17), function 1708 is subtractedfrom function 1706. The autocorrelation terms of the reference signalA1(t) and echo signal A2(t) are squared, positive terms; thus, theycancel out in the subtraction. The cross-correlation terms add (e.g.,[C(t)−(−C(t)]). As shown in FIG. 17, a resultant function 1712 isproduced from the subtraction step 1710. Note from function 1712 that:(1) the zero time delay peak, contributed mainly by the A1(t) term, issubstantially removed; (2) the correlation noise floor is reduced; and(3) the cross-correlation term C(t) (remaining peak) is more prominentfor detection.

Power for CPD generator 1600 is obtained from the associated remote PHYdevice via a power output port on the device. The power from the remotePHY device is usually +24 VDC. A power cable is connected between theoutput port of the remote PHY device and power input port 1616. In someremote PHY devices, a combined RF and power output port may be availableto permit power to be received through coupler element 1602. In CPDgenerator 1600, a power supply (not shown) will divide the voltage downto appropriate values for powering the active circuits, such as pulsemodulator 1610, RF switch 1613, and the FPGA chip (controller 1614).

In the third embodiment, the preferred accumulation time for thecombined autocorrelation is 800 milliseconds. This accumulation time canbe established by setting the duration of each pulse 1700 to 800milliseconds. The blank interval between pulses 1700 (e.g., betweenpulses 1702 and 1704) is used in the CPD core to trigger the end of oneautocorrelation and the beginning of the next autocorrelation (atopposite phase). CPD echo signal samples will be received during theblank interval between pulses; however, the stronger reference CPDsignal will not be receiving during this interval. Thus, the CPD corecan recognize the interval between pulses and use it as a controlmechanism or trigger.

In the second and third (active) embodiments of the CPD generator, anamplifier with automatic gain control (AGC) may be inserted after thehighpass filter leg of the diplex filter (1404/1604) and before the CPDsimulator circuit (1406/1606), to provide better isolation and signallevel stability of the forward signal before IM production. Also, anattenuator (as in the passive embodiment) may be inserted between theAGC amp and the CPD simulator circuit.

In all CPD generator embodiments, the level of the reference CPD signalshould be higher than the noise floor in the return channel at theremote PHY device. In the first (passive) and third embodiments, thelevel should be as low as possible to avoid interfering with the actualCPD echo signal. The expected maximum level of the actual CPD echosignal is about 20 dB below the carrier level of an upstream QAM signal(i.e., −20 dBc). As a result of computer simulations, it has beenconcluded that (for the first and third embodiments) the minimum levelof the reference CPD signal should be about the same as the expectedmaximum level of the actual CPD echo signal (or −20 dBc). The typicalupstream QAM signal level at the input of the remote PHY device is about+15 dBmV, so, in such case, a minimum level for the reference CPD signalwould be about −5 dBmV. If a 30 dBc carrier-to-noise ratio (CNR) isassumed in the upstream, then the noise floor is about −15 dBmV. Thus,the minimum level of the reference CPD signal is about 10 dB abovenoise, and the preferred range is about 10-20 dB above noise.

Also, it is desirable to detect the CPD echo signal within the range ofabout −20 dBc to about −60 dBc. Thus, the dynamic range of the CPDdetection process should be about 40 dB or more. This dynamic rangeshould be achievable with a reference CPD signal of at least −5 dBmV andan autocorrelation accumulation time of 800 milliseconds. Note again, inthe case of the second embodiment (FIG. 14), the level of the referenceCPD signal should be increased (without concern of channel interference)in order to provide higher fidelity reference samples.

Referring now to FIG. 18, a flow diagram of a method 1800 of theinvention is shown. Method 1800 enables synchronous capture of a forwardsignal and a related actual CPD signal, at a remote PHY device, for thepurpose of detecting CPD and locating its source. The remote PHY deviceis coupled to and delivers the forward signal to a coaxial cable plantof an HFC network. The actual CPD signal is generated by an interactionbetween the forward signal and a CPD source in the coaxial cable plant.The forward signal propagates from the remote PHY device to the CPDsource and the actual CPD signal propagates from the CPD source to theremote PHY device, all within a round-trip interval. In a first step1802 of method 1800, the forward signal is captured from an output portassociated with the remote PHY device, substantially at the beginning ofthe round-trip interval. In a second step 1804, a reference CPD signal,which substantially simulates the actual CPD signal, is generated fromthe forward signal captured in step 1802. In a third step 1806, thereference CPD signal is supplied to the remote PHY device. In a fourthstep 1808, the reference CPD signal is received in the remote PHY devicein a return channel. In a fifth step 1810, the actual CPD signal iscaptured in the remote PHY device, in the return channel, substantiallyat the end of the round-trip interval. The reference and actual CPDsignals are processed as a combined return signal.

In a sixth step 1812 of method 1800, the combined return signal istransmitted from the remote PHY device to a headend of the HFC network.In a seventh step 1814, at the headend or a device communicating withthe headend (e.g., CPD core), a cross-correlation of the reference andactual CPD signals of the combined return signal is performed, toproduce a correlation peak having a time delay corresponding to theround-trip interval. Step 1814 is preferably performed by firstcomputing the autocorrelation function of the combined return signal andthen extracting the cross-correlation term(s) from the function. In aneighth step 1816, the actual CPD signal is detected from the correlationpeak. Lastly, in a ninth step 1818, a location of the CPD source in thecoaxial cable plant is determined from the time delay of the correlationpeak.

Referring now to FIG. 19, there is shown a block diagram of a remote PHYnode 1908, which is to be discussed in connection with a fourthembodiment of the present invention. Node 1908 has a similar structureas remote PHY node 108 in FIG. 1, except as herein described. In FIG.19, like reference numerals refer to like parts with respect to FIG. 1.Node 1908 contains a chipset unit 1922, a forward amplifier 1924, areturn amplifier 1926, a diplexer 1928, and an output tap 1930containing an output test port 1931. Chipset unit 1922 includes aforward signal transmitter chipset 1922 a and a return signal receiverchipset 1922 b. Chipset 1992 a contains downstream QAM modulators andchipset 1922 b contains upstream QAM demodulators, and both containlogic to connect to and communicate with a CCAP core at a headend.Diplexer 1928 includes a forward leg 1928 a and a return leg 1928 b. Aphysical forward path 1925 is defined between forward signal transmitterchipset 1922 a and forward leg 1928 a. A physical return path 1927 isdefined between return signal receiver chipset 1922 b and return leg1928 b. A downstream or forward signal produced in transmitter chipset1922 a is amplified by forward amp 1924, passes through forward leg 1928a of diplexer 1928, passes through tap 1930, exits node 1908 at an RFoutput port 1930 a, and then travels to a coaxial cable plant (notshown). Upstream or return signals from the coaxial cable plant travelto and are received by node 1908. The return signals pass through returnleg 1928 b of diplexer 1928, are amplified by return amp 1926, receivedby receiver chipset 1922 b for demodulation and down-conversion, andthen communicated to a CCAP core at a headend of an HFC network. Outputtest port 1931 is terminated and not used in the fourth embodiment. Anadvantage of the fourth embodiment is that a CPD generator unit is notused to accomplish synchronous capture of the forward signal.

Signal isolation between forward leg 1928 a and return leg 1928 b is notperfect; in fact, it can be as low as 40 dB of attenuation. Thus,inevitably, there is leakage of the forward signal from forward leg 1928a to return leg 1928 b (see Leakage arrow in FIG. 19). A leaked portionof the forward signal travels from return leg 1928 b, over physicalreturn path 1927, to return signal receiver 1922 b. The leaked portionof the forward signal (plus system noise at forward signal frequencies)is amplified by return amplifier 1926. This phenomenon of forward signalleakage is the basis of the fourth embodiment, in that the leakageprovides access to the forward signal (at least a portion of itsspectrum) at receiver 1922 b for synchronous capture along with actualCPD echo signals. In accordance with the fourth embodiment, receiver1922 b is operated in such a manner as to take advantage of (i.e.,capture) the available leaked portion of the forward signal. Thus, thereis no need to capture the forward signal in a CPD generator devicecoupled to output test port 1931.

In order to take advantage of the leaked forward signal, return signalreceiver 1922 b should be flexible in its operating parameters. Thebandwidth over which it can capture signals (“capturing bandwidth”)should be adjustable and the center frequency of the capturing bandwidthshould also be adjustable. By opening or enlarging the capturingbandwidth and centering the bandwidth at a suitable frequency, asufficient amount of the forward signal spectrum (along with the returnsignal spectrum) can be captured (sufficient enough for later generatinga reference CPD signal to be used in cross-correlation detection andlocation of CPD). The frequency spectrum of the forward signal isreferred to herein as the forward signal spectrum (i.e., a band offrequencies). A suitable chipset for receiver 1922 b is the modelBCM31442 chip, manufactured by Broadcom, Inc., San Jose, Calif. TheBCM31442 chip has a frequency span or maximum capturing bandwidth of409.6 MHz with selectable center frequencies of 128 MHz, 179.2 MHz,230.4 MHz, or 281.6 MHz. Thus, if receiver 1922 b is configured to havea center frequency of 230.4 MHz and a capturing bandwidth of 409.6 MHz,receiver 1922 b will be able to capture, for example, a return signalfrom 25 (about) to 85 MHz and a forward signal from 100 to 435.2 MHz.Thus, the range of frequencies of the forward signal spectrum that canbe captured by receiver 1922 b is 335.2 MHz (435.2−100), which is morethan enough to generate a high fidelity reference CPD signal for CPDdetection. Both the center frequency and the frequency span can beprogrammable from a CPD core (e.g., CPD core 118 in FIG. 1) via a CCAPcore (e.g., CCAP core 104 in FIG. 1). The captured spectrum data is sentfrom receiver 1922 b to the CCAP core and then to the CPD core forgeneration of the reference CPD signal and cross-correlation detectionwith the actual CPD echo signal (captured as part of the return signalsin the return spectrum).

FIG. 20 is an image of a spectrum 2000 captured by receiver 1922 b.Spectrum 2000 is a plot of signal level in dBmV versus frequency in MHz.In this example, the settings in receiver 1922 b included a centerfrequency of 128 MHz and a span of 409.6 MHz. However, because of thelocation of the center frequency, the captured bandwidth (and capturingbandwidth) is only 330 MHz. The captured bandwidth (spectrum 2000)includes an entire return spectrum 2002, from 5 to 85 MHz, and a rangeof frequencies 2004 of the forward spectrum, from 100 to 330 MHz(about). Range of frequencies 2004 is present in spectrum 2000 due tothe limited isolation in diplexer 1928 (leakage). As shown, the signalsin range 2004 have levels as high as plus-or-minus −30 dBmV, which issufficient to generate the reference CPD signal for cross-correlationdetection.

The duration of the signals captured by receiver 1922 b is 10 or 20microseconds. In the case of 20 microseconds, the frequency span mayneed to be reduced from 409.6 MHz to 208.4 MHz. This will result in areduced range of frequencies that can be captured of the forward signalspectrum. However, the reduced range will be sufficient to produce asuitable reference CPD signal. One test showed that only 8 QAM channels(48 MHz) produced a suitable reference CPD signal for detection of CPDwith an adequate signal-to-noise ratio. Due to the limitation on thesignal duration, the range of distances (or time distances) of possibleCPD sources (from the remote PHY node) may be limited to 8 microseconds(16 microseconds round-trip interval) or approximately 1000 meters (2000round-trip distance).

Receiver 1922 b (and the BCM31442 chip) allows the spectrum data to becaptured as: (1) samples of a Fast Fourier Transform (FFT) processor(generated after baseband down-conversion of the signal) in receiver1922 b; or (2) the direct output of an analog-to-digital converter (ADC)inside receiver 1922 b. In case (1), the bit rate the FFT (4K) isapproximately 100 Mbps according measured results. In case of aquadrature I/Q data, the traffic will be twice this, or approximately200 Mbps. This data traffic is only about twice the traffic generated inthe other embodiments using the 5.12 MHz NDR channel; and, the datatraffic in the fourth embodiment will only be required for a shortperiod of time (e.g., a few seconds). Thus, the increased data trafficshould not impact normal operation of the network system.

Referring again to FIG. 19, diplexer 1928 is not limited to a diplexfilter embodiment, as is diplex filter 128 in FIG. 1. Diplexer 1928symbolizes any combining and splitting network (e.g., an impedancebridge, a signal splitter, or diplex filter). In further networksystems, employing full duplex operation, diplexer 1928 will be a signalsplitter. In current remote PHY devices, diplexer 1928 represents adiplex filter, where forward leg 1928 a is a highpass filter leg andreturn leg 1928 b is lowpass filter leg (as described with respect todiplex filter 128 in FIG. 1). The term “diplexer” as used in thisspecification and the claims is intended to encompass any such combiningand splitting network (diplex filter, signal splitter, etc.).

The above-mentioned BCM31442 chip, by Broadcom, is an advanced DOCSIS®3.1 dual-port cable network burst receiver that accepts upstream burstdata in a frequency-agile time division multiple access (TDMA),synchronous code division multiple access (S-CDMA), or orthogonalfrequency division multiple access (OFDMA) scheme. The chip integratesan analog front end, QAM (up to 4096 order)/OFDMA demodulators, FECdecoders, and a MAC/PHY interface. The analog front end performsanalog-to-digital (A/D) conversion on RF inputs with a carrier frequencyof up to 300 MHz; the integrated A/D converters support direct digitalsampling of the entire 0 to 300 MHz reverse path. A digital quadraturedown-mixer translates the input spectral center to baseband. TheBCM31442 delivers the recovered data streams and accepts receivercontrol inputs through a receive interface linked to a MAC chip (in CCAPcore). An on-chip FFT processor can analyze an RF spectrum with aselectable bandwidth and length; it enables advanced spectrum managementof the entire 0 to 300 MHz upstream.

Forward signal transmitter 1922 a may be a Broadcom model BCM3047 singleport downstream modulator chip with support for 160 QAM narrowcastchannels and additional support for six OFDM 192 MHz channels. TheBCM3047 has an integrated 6.4 GHz DAC for direct digital synthesis up to1.2 GHz. The BCM3047 is CCAP-compliant and supports DOCSIS®1.0/1.1/2.0/3.0/3.1 specifications and SCTE/ITU-B standards. Each QAMdownstream modulator in the chip supports programmable modulationformats up to 1024 QAM (per the C-DOCSIS standard). The chip alsoincludes SCTE 55-1 and SCTE 55-2 out-of-band (OOB) channel modulators.

Referring now to FIG. 21, a flow diagram of a method 2100 of theinvention is shown (fourth embodiment). Method 2100 is a method ofsynchronous capture of a forward signal and a related actual common pathdistortion (CPD) signal at a remote physical layer (PHY) device, for thepurpose of detecting CPD and locating its source. The remote PHY deviceis coupled between a headend and a coaxial cable plant of an HFC networkand includes a forward signal transmitter, a return signal receiver, anda diplexer having forward and return legs. A physical forward path isdefined between the forward signal transmitter and the forward leg, anda physical return path is defined between the return signal receiver andthe return leg. The forward signal transmitter transmits the forwardsignal to the coaxial cable plant via the physical forward path and theforward leg of the diplexer. A portion of the forward signal leaksthrough to the return leg of the diplexer and travels over the physicalreturn path to the return signal receiver. The actual CPD signal isgenerated by an interaction between the forward signal and a CPD sourcein the coaxial cable plant. The forward signal propagates from theremote PHY device to the CPD source and the actual CPD signal propagatesfrom the CPD source to the remote PHY device all within a round-tripinterval. Method 2100 comprises steps 2102 through 2114.

In a first step 2102, return signal receiver 1922 b of remote PHY node1908 is configured to capture a maximal range of frequencies of theforward signal spectrum. This may be accomplished by instructions fromthe CPD core delivered to the CCAP core and then delivered to receiver1922 b. In a second step 2104, receiver 1922 b is operated to capturethe actual CPD signal (in the return spectrum) and the leaked portion ofthe forward signal over a duration of at least the round-trip interval.In a third step 2106, the actual CPD signal and the leaked portion ofthe forward signal (captured in step 2104) is transmitted to the headendof the HFC network. For example, it is transmitted from receiver 1922 bto the CCAP core and then to the CPD core. In a fourth step 2108, at theheadend or at a device that communicates with the headend (e.g., CPDcore), a reference CPD signal (which substantially simulates the actualCPD signal) is generated from the leaked portion of the forward signal.In a fifth step 2110, a cross-correlation of the reference and actualCPD signals is performed to produce a correlation peak having a timedelay corresponding to the round-trip interval. In a sixth step 2112,the actual CPD signal is detected from the correlation peak. Finally, ina seventh step 2114, a location of the CPD source in the coaxial cableplant (relative to the remote PHY device) is determined from the timedelay of the correlation peak.

The above-described functions of the invention may be implemented inhardware, firmware, software, or a combination of these. If in hardware,the functions may be implemented in an application specific integratedcircuit (ASIC), digital signal processor (DSP), FPGA, micro-controller,microprocessor, programmable logic device, general purpose computer,special purpose computer, other electronic device, or a combination ofthese. If the functions are implemented in firmware or software, thenthey may be stored on computer-readable media. Computer-executableinstructions may cause hardware to perform the functions of theinvention. Such instructions include data structures, objects, programs,routines, or other program modules. Computer-readable media include, butare not limited to, random-access memory (“RAM), read-only memory(“ROM), programmable read-only memory (“PROM), erasable programmableread-only memory (“EPROM), electrically erasable programmable read-onlymemory (“EEPROM”), compact disk read-only memory (“CD-ROM), or otherdevice or component capable of providing data or executableinstructions. Certain claimed embodiments may be limited to the use oftangible, non-transitory computer-readable media, and the phrases“tangible computer-readable medium” and “non-transitorycomputer-readable medium” (or plural variations) used herein areintended to exclude transitory propagating signals per se.

While the preferred embodiments of the invention have been describedherein, it should be understood that the invention is not so limited.Many modifications, equivalents and adaptations of the invention willbecome apparent to those skilled in the art without departing from thespirit and scope of the invention, as defined in the appended claims.

What is claimed is:
 1. An apparatus for enabling synchronous capture offorward and return signals at a remote physical layer (PHY) device forthe purpose of detecting common path distortion (CPD), the remote PHYdevice being coupled between a headend and a coaxial cable plant of ahybrid-fiber coax (HFC) network and providing the forward signal to andreceiving the return signal from the coaxial cable plant, the returnsignal containing an actual CPD signal generated by an interactionbetween the forward signal and a CPD source in the coaxial cable plant,the forward signal propagating from the remote PHY device to the CPDsource and the actual CPD signal propagating from the CPD source to theremote PHY device all within a round-trip interval, the remote PHYdevice capturing the return signal, from a return channel, over acapture duration of at least the round-trip interval, said apparatuscomprising: (a) a coupling element, coupleable to the remote PHY device,for capturing the forward signal from the remote PHY device; (b) a CPDsimulator circuit, coupled to said coupling element, for generating fromthe forward signal a simulated CPD signal having frequencies in thereturn channel; and (c) a filter, coupled to said CPD simulator circuit,for substantially band-limiting the simulated CPD signal to the returnchannel, to produce a reference CPD signal, and a highpass filter,coupled between said coupling element and said CPD simulator circuit,having a passband and a reject band separated by a cutoff frequency, thepassband being configured to substantially pass the forward signal tosaid CPD simulator circuit and the reject band being configured tosubstantially reject frequencies of the simulated CPD signal lower thanthe cutoff frequency; and said filter being further coupled to saidcoupling element to supply the reference CPD signal to the remote PHYdevice, such that the reference CPD signal is received by the remote PHYdevice in the return channel, whereby the reference CPD signal isavailable for use in detecting the actual CPD signal from the capturedreturn signal.
 2. The apparatus as recited in claim 1, wherein thereturn channel is an out-of-band return channel.
 3. The apparatus asrecited in claim 2, wherein the out-of-band return channel is anarrowband digital return (NDR) channel.
 4. The apparatus as recited inclaim 1, further comprising an attenuator circuit, coupled between saidhighpass filter and said CPD simulator circuit, for attenuating thelevel of the forward signal as it passes to said CPD simulator circuit.5. The apparatus as recited in claim 1, further comprising a housingcontaining said coupling element, said CPD simulator circuit, and saidfilter, said coupling element including a cable connector associatedwith said housing and configured to physically and electrically connectto a complementary connector associated with the remote PHY device. 6.An apparatus for enabling synchronous capture of forward and returnsignals at a remote physical layer (PHY) device for the purpose ofdetecting common path distortion (CPD), the remote PHY device beingcoupled between a headend and a coaxial cable plant of a hybrid-fibercoax (HFC) network and providing the forward signal to and receiving thereturn signal from the coaxial cable plant, the return signal containingan actual CPD signal generated by an interaction between the forwardsignal and a CPD source in the coaxial cable plant, the forward signalpropagating from the remote PHY device to the CPD source and the actualCPD signal propagating from the CPD source to the remote PHY device allwithin a round-trip interval, the remote PHY device capturing the returnsignal, from a return channel, over a capture duration of at least theround-trip interval, said apparatus comprising: (a) a coupling element,coupleable to the remote PHY device, for capturing the forward signalfrom the remote PHY device; (b) a CPD simulator circuit, coupled to saidcoupling element, for generating from the forward signal a simulated CPDsignal having frequencies in the return channel; and (c) a filter,coupled to said CPD simulator circuit, for substantially band-limitingthe simulated CPD signal to the return channel, to produce a referenceCPD signal, (d) a pulse modulation circuit, coupled to said filter, forshaping the reference CPD signal into a pulse; and (e) a time delayline, coupled to said pulse modulation circuit, for delaying thepulse-shaped reference CPD signal such that the return signal iscaptured in the remote PHY device without substantial interference fromthe received pulse-shaped reference CPD signal, said filter beingfurther coupled to said coupling element to supply the reference CPDsignal to the remote PHY device, such that the reference CPD signal isreceived by the remote PHY device in the return channel, whereby thereference CPD signal is available for use in detecting the actual CPDsignal from the captured return signal.
 7. The apparatus as recited inclaim 6, wherein said time delay line is configured to delay thepulse-shaped reference CPD signal longer than the round-trip interval.8. An apparatus for enabling synchronous capture of forward and returnsignals at a remote physical layer (PHY) device for the purpose ofdetecting common path distortion (CPD), the remote PHY device beingcoupled between a headend and a coaxial cable plant of a hybrid-fibercoax (HFC) network and providing the forward signal to and receiving thereturn signal from the coaxial cable plant, the return signal containingan actual CPD signal generated by an interaction between the forwardsignal and a CPD source in the coaxial cable plant, the forward signalpropagating from the remote PHY device to the CPD source and the actualCPD signal propagating from the CPD source to the remote PHY device allwithin a round-trip interval, the remote PHY device capturing the returnsignal, from a return channel, over a capture duration of at least theround-trip interval, said apparatus comprising: (a) a coupling element,coupleable to the remote PHY device, for capturing the forward signalfrom the remote PHY device; (b) a CPD simulator circuit, coupled to saidcoupling element, for generating from the forward signal a simulated CPDsignal having frequencies in the return channel; and (c) a filter,coupled to said CPD simulator circuit, for substantially band-limitingthe simulated CPD signal to the return channel, to produce a referenceCPD signal, (d) a pulse modulation circuit, coupled to said filter, forshaping the reference CPD signal into a first pulse and shaping a secondreference CPD signal produced by said filter into a second pulse; and(e) a phase inverter circuit, coupled to said pulse modulation circuit,for shifting the phase of the second pulse by about 180 degrees, wherebyinterference caused by an autocorrelation of the first pulse, indetecting the actual CPD signal, is substantially cancelable byinterference caused by an autocorrelation of the second pulse saidfilter being further coupled to said coupling element to supply thereference CPD signal to the remote PHY device, such that the referenceCPD signal is received by the remote PHY device in the return channel,whereby the reference CPD signal is available for use in detecting theactual CPD signal from the captured return signal.
 9. An apparatus forenabling synchronous capture of forward and return signals at a remotephysical layer (PHY) device for the purpose of detecting common pathdistortion (CPD), the remote PHY device being coupled between a headendand a coaxial cable plant of a hybrid-fiber coax (HFC) network andproviding the forward signal to and receiving the return signal from thecoaxial cable plant, the return signal containing an actual CPD signalgenerated by an interaction between the forward signal and a CPD sourcein the coaxial cable plant, the forward signal propagating from theremote PHY device to the CPD source and the actual CPD signalpropagating from the CPD source to the remote PHY device all within around-trip interval, the remote PHY device capturing the return signal,from a return channel, over a capture duration of at least theround-trip interval, said apparatus comprising: (a) a coupling element,coupleable to the remote PHY device, for capturing the forward signalfrom the remote PHY device; (b) a CPD simulator circuit, coupled to saidcoupling element, for generating from the forward signal a simulated CPDsignal having frequencies in the return channel; and (c) a filter,coupled to said CPD simulator circuit, for substantially band-limitingthe simulated CPD signal to the return channel, to produce a referenceCPD signal, said filter being further coupled to said coupling elementto supply the reference CPD signal to the remote PHY device, such thatthe reference CPD signal is received by the remote PHY device in thereturn channel, whereby the reference CPD signal is available for use indetecting the actual CPD signal from the captured return signal, andwherein the reference CPD signal, as received by the remote PHY device,has a signal level in a range of about 0 dB to about 5 dB relative to anexpected maximum level of the actual CPD signal contained in thecaptured return signal.
 10. A method of enabling synchronous capture offorward and return signals at a remote physical layer (PHY) device forthe purpose of detecting common path distortion (CPD), the remote PHYdevice being coupled between a headend and a coaxial cable plant of ahybrid-fiber coax (HFC) network and providing the forward signal to andreceiving the return signal from the coaxial cable plant, the returnsignal containing an actual CPD signal generated by an interactionbetween the forward signal and a CPD source in the coaxial cable plant,the forward signal propagating from the remote PHY device to the CPDsource and the actual CPD signal propagating from the CPD source to theremote PHY device all within a round-trip interval, said methodcomprising the steps of: (a) capturing the forward signal from an outputassociated with the remote PHY device; (b) generating from the forwardsignal captured in step (a) a simulated CPD signal having frequencies ina return channel; (c) substantially band-limiting the simulated CPDsignal to the return channel, to produce a reference CPD signal; (d)supplying the reference CPD signal to the remote PHY device; (e) in theremote PHY device, receiving the reference CPD signal in the returnchannel; and (f) in the remote PHY device, capturing the return signalfrom the return channel over a capture duration of at least theround-trip interval, (g) transmitting the reference CPD signal and thecaptured return signal from the remote PHY device to the headend of theHFC network; (h) at the headend or a device communicating with theheadend, performing a cross-correlation of the reference CPD signal andthe captured return signal, to produce a correlation peak associatedwith the actual CPD signal; and (i) detecting the actual CPD signal fromthe correlation peak, whereby the reference CPD signal is available foruse in detecting the actual CPD signal from the captured return signal.11. The method as recited in claim 10, further comprising the step of:(g) detecting the actual CPD signal from the captured return signalusing the reference CPD signal.
 12. The method as recited in claim 10,wherein step (d) includes supplying the reference CPD signal to theremote PHY device via the output associated with the remote PHY device.13. The method as recited in claim 10, wherein the reference CPD signaland the captured return signal are processed as a combined signal, andwherein step (h) includes performing an autocorrelation of the combinedsignal to obtain the cross-correlation of the reference CPD signal andthe captured return signal.
 14. The method as recited in claim 13,wherein the autocorrelation of the combined signal includes anautocorrelation of the reference CPD signal, and wherein step (h)further includes substantially subtracting the autocorrelation of thereference CPD signal from the autocorrelation of the combined signal.15. The method as recited in claim 13, wherein the combined signal has aduration of about 100 milliseconds or greater; and wherein step (h)includes performing the autocorrelation of the combined signalsubstantially over the duration of the combined signal.
 16. A method ofenabling synchronous capture of forward and return signals at a remotephysical layer (PHY) device for the purpose of detecting common pathdistortion (CPD), the remote PHY device being coupled between a headendand a coaxial cable plant of a hybrid-fiber coax (HFC) network andproviding the forward signal to and receiving the return signal from thecoaxial cable plant, the return signal containing an actual CPD signalgenerated by an interaction between the forward signal and a CPD sourcein the coaxial cable plant, the forward signal propagating from theremote PHY device to the CPD source and the actual CPD signalpropagating from the CPD source to the remote PHY device all within around-trip interval, said method comprising the steps of: (a) capturingthe forward signal from an output associated with the remote PHY device;(b) generating from the forward signal captured in step (a) a simulatedCPD signal having frequencies in a return channel; (c) substantiallyband-limiting the simulated CPD signal to the return channel, to producea reference CPD signal; (d) supplying the reference CPD signal to theremote PHY device; (e) in the remote PHY device, receiving the referenceCPD signal in the return channel; and (f) in the remote PHY device,capturing the return signal from the return channel over a captureduration of at least the round-trip interval, whereby the reference CPDsignal is available for use in detecting the actual CPD signal from thecaptured return signal, and wherein the reference CPD signal and thecaptured return signal are processed as a combined signal, said methodfurther comprising the steps of: (g) performing a first combinedautocorrelation of the combined signal, which includes a first referenceautocorrelation of the reference CPD signal and a firstcross-correlation of the reference CPD signal and the captured returnsignal; (h) repeating steps (a), (b), and (c) to generate a secondreference CPD signal; (i) shifting the phase of the second reference CPDsignal by about 180 degrees; (j) repeating steps (d) through (e) withrespect to the phase-shifted second reference CPD signal, and repeatingstep (f) to capture a second return signal, the phase-shifted secondreference CPD signal and the second return signal being processed as asecond combined signal; (k) performing a second combined autocorrelationof the second combined signal, which includes a second referenceautocorrelation of the second reference CPD signal and a secondcross-correlation of the second reference CPD signal and the secondreturn signal; (l) subtracting the second combined autocorrelation fromthe first combined autocorrelation, such that the first and the secondreference autocorrelations substantially cancel each other out, and thefirst and the second cross-correlations substantially add together toproduce an accumulated cross-correlation function having a correlationpeak; and (m) detecting the actual CPD signal from the correlation peak.17. The method as recited in claim 16, wherein step (c) includes shapingthe reference CPD signal into a pulse, and wherein step (h) includesshaping the second reference CPD signal into a pulse.
 18. A method ofenabling synchronous capture of forward and return signals at a remotephysical layer (PHY) device for the purpose of detecting common pathdistortion (CPD), the remote PHY device being coupled between a headendand a coaxial cable plant of a hybrid-fiber coax (HFC) network andproviding the forward signal to and receiving the return signal from thecoaxial cable plant, the return signal containing an actual CPD signalgenerated by an interaction between the forward signal and a CPD sourcein the coaxial cable plant, the forward signal propagating from theremote PHY device to the CPD source and the actual CPD signalpropagating from the CPD source to the remote PHY device all within around-trip interval, said method comprising the steps of: (a) capturingthe forward signal from an output associated with the remote PHY device;(b) generating from the forward signal captured in step (a) a simulatedCPD signal having frequencies in a return channel; (c) substantiallyband-limiting the simulated CPD signal to the return channel, to producea reference CPD signal; (d) supplying the reference CPD signal to theremote PHY device; (e) in the remote PHY device, receiving the referenceCPD signal in the return channel; and (f) in the remote PHY device,capturing the return signal from the return channel over a captureduration of at least the round-trip interval, whereby the reference CPDsignal is available for use in detecting the actual CPD signal from thecaptured return signal, and (g) shaping the reference CPD signal into apulse; and (h) delaying the pulse-shaped reference CPD signal such thatthe return signal is captured in step (f) without substantialinterference from the pulse-shaped reference CPD signal.
 19. The methodas recited in claim 18 wherein step (h) includes delaying thepulse-shaped reference CPD signal longer than the round-trip interval.20. A method of enabling synchronous capture of forward and returnsignals at a remote physical layer (PHY) device for the purpose ofdetecting common path distortion (CPD), the remote PHY device beingcoupled between a headend and a coaxial cable plant of a hybrid-fibercoax (HFC) network and providing the forward signal to and receiving thereturn signal from the coaxial cable plant, the return signal containingan actual CPD signal generated by an interaction between the forwardsignal and a CPD source in the coaxial cable plant, the forward signalpropagating from the remote PHY device to the CPD source and the actualCPD signal propagating from the CPD source to the remote PHY device allwithin a round-trip interval, said method comprising the steps of: (a)capturing the forward signal from an output associated with the remotePHY device; (b) generating from the forward signal captured in step (a)a simulated CPD signal having frequencies in a return channel; (c)substantially band-limiting the simulated CPD signal to the returnchannel, to produce a reference CPD signal; (d) supplying the referenceCPD signal to the remote PHY device; (e) in the remote PHY device,receiving the reference CPD signal in the return channel; and (f) in theremote PHY device, capturing the return signal from the return channelover a capture duration of at least the round-trip interval, whereby thereference CPD signal is available for use in detecting the actual CPDsignal from the captured return signal, and wherein the return channelhas a noise floor, and wherein step (e) includes receiving the referenceCPD signal in the remote PHY device at a level of about 10 dB to about20 dB above the noise floor.